{"id":252,"date":"2021-07-14T15:58:59","date_gmt":"2021-07-14T15:58:59","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/introstatscorequisite\/chapter\/the-uniform-distribution\/"},"modified":"2023-12-05T09:15:20","modified_gmt":"2023-12-05T09:15:20","slug":"the-uniform-distribution","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/introstatscorequisite\/chapter\/the-uniform-distribution\/","title":{"raw":"What is a Uniform Distribution?","rendered":"What is a Uniform Distribution?"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Calculate the mean and standard deviation for a uniform distribution<\/li>\r\n \t<li>Calculate probabilities, including conditional probabilities, for a uniform distribution<\/li>\r\n \t<li>Calculate percentiles for a uniform distribution<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe <strong>uniform distribution<\/strong> is a continuous probability distribution and is concerned with events that are equally likely to occur. When working out problems that have a uniform distribution, be careful to note if the data is inclusive or exclusive.\r\n<div class=\"textbox exercises\">\r\n<h3>Example<\/h3>\r\nThe data in the table below are 55 smiling times, in seconds, of an eight-week-old baby.\r\n<table>\r\n<tbody>\r\n<tr>\r\n<td scope=\"row\">10.4<\/td>\r\n<td>19.6<\/td>\r\n<td>18.8<\/td>\r\n<td>13.9<\/td>\r\n<td>17.8<\/td>\r\n<td>16.8<\/td>\r\n<td>21.6<\/td>\r\n<td>17.9<\/td>\r\n<td>12.5<\/td>\r\n<td>11.1<\/td>\r\n<td>4.9<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">12.8<\/td>\r\n<td>14.8<\/td>\r\n<td>22.8<\/td>\r\n<td>20.0<\/td>\r\n<td>15.9<\/td>\r\n<td>16.3<\/td>\r\n<td>13.4<\/td>\r\n<td>17.1<\/td>\r\n<td>14.5<\/td>\r\n<td>19.0<\/td>\r\n<td>22.8<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">1.3<\/td>\r\n<td>0.7<\/td>\r\n<td>8.9<\/td>\r\n<td>11.9<\/td>\r\n<td>10.9<\/td>\r\n<td>7.3<\/td>\r\n<td>5.9<\/td>\r\n<td>3.7<\/td>\r\n<td>17.9<\/td>\r\n<td>19.2<\/td>\r\n<td>9.8<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">5.8<\/td>\r\n<td>6.9<\/td>\r\n<td>2.6<\/td>\r\n<td>5.8<\/td>\r\n<td>21.7<\/td>\r\n<td>11.8<\/td>\r\n<td>3.4<\/td>\r\n<td>2.1<\/td>\r\n<td>4.5<\/td>\r\n<td>6.3<\/td>\r\n<td>10.7<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">8.9<\/td>\r\n<td>9.4<\/td>\r\n<td>9.4<\/td>\r\n<td>7.6<\/td>\r\n<td>10.0<\/td>\r\n<td>3.3<\/td>\r\n<td>6.7<\/td>\r\n<td>7.8<\/td>\r\n<td>11.6<\/td>\r\n<td>13.8<\/td>\r\n<td>18.6<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nThe sample mean[latex]=11.49[\/latex] and the sample standard deviation[latex]=6.23[\/latex].\r\n\r\nWe will assume that the smiling times, in seconds, follow a uniform distribution between zero and 23 seconds, inclusive. This means that any smiling time from zero to and including 23 seconds is <strong>equally likely<\/strong>. The histogram that could be constructed from the sample is an empirical distribution that closely matches the theoretical uniform distribution.\r\n\r\nLet [latex]X=[\/latex] length, in seconds, of an eight-week-old baby's smile.\r\n\r\nThe notation for the uniform distribution is [latex]X{\\sim}U(a,b)[\/latex] where [latex]a=[\/latex] the lowest value of [latex]x[\/latex]\u00a0and\u00a0[latex]b=[\/latex] the highest value of [latex]x[\/latex].\r\n\r\nThe probability density function is [latex]{f{{({x})}}}=\\frac{{1}}{{{b}-{a}}}[\/latex] for [latex]a{\\leq}x{\\leq}b[\/latex].\r\n\r\nFor this example, [latex]X{\\sim}U(0,23)[\/latex] and [latex]{f{{({x})}}}=\\frac{{1}}{{{23}-{0}}}[\/latex] for [latex]0{\\leq}X{\\leq}23[\/latex].\r\n\r\nFormulas for the theoretical mean and standard deviation are [latex]{\\mu}=\\frac{{{a}+{b}}}{{2}}{\\quad\\text{and}\\quad}{\\sigma}=\\sqrt{{\\frac{{{({b}-{a})}^{{2}}}}{{12}}}}[\/latex]\r\n\r\nFor this problem, the theoretical mean and standard deviation are [latex]{\\mu}=\\frac{{{0}+{23}}}{{2}}={11.50} \\text{ seconds}{\\quad\\text{and}\\quad}{\\sigma}=\\sqrt{{\\frac{{{({23}-{0})}^{{2}}}}{{12}}}}={6.64} \\text{ seconds}[\/latex]\r\n\r\nNotice that the theoretical mean and standard deviation are close to the sample mean and standard deviation in this example.\r\n\r\n<\/div>\r\n\r\n<hr \/>\r\n\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Try It<\/h3>\r\nThe data that follow are the number of passengers on 35 different charter fishing boats. The sample mean [latex]=7.9[\/latex] and the sample standard deviation [latex]=4.33[\/latex]. The data follow a uniform distribution where all values between and including zero and 14 are equally likely. State the values of\u00a0<em>a<\/em> and <em>b<\/em>. Write the distribution in proper notation, and calculate the theoretical mean and standard deviation.\r\n<table>\r\n<tbody>\r\n<tr>\r\n<td scope=\"row\">1<\/td>\r\n<td>12<\/td>\r\n<td>4<\/td>\r\n<td>10<\/td>\r\n<td>4<\/td>\r\n<td>14<\/td>\r\n<td>11<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">7<\/td>\r\n<td>11<\/td>\r\n<td>4<\/td>\r\n<td>13<\/td>\r\n<td>2<\/td>\r\n<td>4<\/td>\r\n<td>6<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">3<\/td>\r\n<td>10<\/td>\r\n<td>0<\/td>\r\n<td>12<\/td>\r\n<td>6<\/td>\r\n<td>9<\/td>\r\n<td>10<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">5<\/td>\r\n<td>13<\/td>\r\n<td>4<\/td>\r\n<td>10<\/td>\r\n<td>14<\/td>\r\n<td>12<\/td>\r\n<td>11<\/td>\r\n<\/tr>\r\n<tr>\r\n<td scope=\"row\">6<\/td>\r\n<td>10<\/td>\r\n<td>11<\/td>\r\n<td>0<\/td>\r\n<td>11<\/td>\r\n<td>13<\/td>\r\n<td>2<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[reveal-answer q=\"810841\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"810841\"]\r\n\r\n<em>a<\/em> is zero;<em> b<\/em> is 14; [latex]X{\\sim}U(0,14)[\/latex]; [latex]\\mu=7[\/latex] passengers; [latex]\\sigma=4.04[\/latex] passengers\r\n\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Example<\/h3>\r\n<ol>\r\n \t<li>Refer to the above example about the eight-week-old baby. What is the probability that a randomly chosen eight-week-old baby smiles between two and 18 seconds?<\/li>\r\n \t<li>Find the 90th percentile for an eight-week-old baby's smiling time.<\/li>\r\n \t<li>Find the probability that a random eight-week-old baby smiles more than 12 seconds knowing that the baby smiles more than eight seconds.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"96316\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"96316\"]\r\n<ol>\r\n \t<li>Find <em>P<\/em>(2 &lt; <em>x<\/em> &lt; 18).\r\n[latex]{P}{({2}{&lt;}{x}{&lt;}{18})}={(\\text{base})}{(\\text{height})}={({18}-{2})}{(\\frac{{1}}{{23}})}={(\\frac{{16}}{{23}})}.[\/latex]<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/80tq-575e627i#fixme#fixme#fixme\" alt=\"This graph shows a uniform distribution. The horizontal axis ranges from 0 to 15. The distribution is modeled by a rectangle extending from x = 0 to x = 15. A region from x = 2 to x = 18 is shaded inside the rectangle.\" \/><\/li>\r\n \t<li>Ninety percent of the smiling times fall below the 90th percentile, <em>k<\/em>, so <em>P<\/em>(<em>x<\/em> &lt;<em>k<\/em>) = 0.90\r\n[latex]P(x&lt;k)=0.90[\/latex]\r\n[latex](\\text{base})(\\text{height})=0.90[\/latex]\r\n[latex](k-0)(\\frac{1}{23})=0.90[\/latex]\r\n[latex]k=(23)(0.90)=20.7[\/latex]\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/5ivo-nc5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/15. A horiztonal line ranges from the point (0, 1\/15) to the point (15, 1\/15). A vertical line extends from the x-axis to the end of the line at point (15, 1\/15) creating a rectangle. A region is shaded inside the rectangle from x = 0 to x = k. The shaded area represents P(x &lt; k) = 0.90.\" \/><\/li>\r\n \t<li>This probability question is a <strong>conditional<\/strong>. You are asked to find the probability that an eight-week-old baby smiles more than 12 seconds when you already know the baby has smiled for more than eight seconds. Find\u00a0<em>P<\/em>(<em>x<\/em> &gt; 12|<em>x<\/em> &gt; 8) There are two ways to do the problem.\r\n<ol>\r\n \t<li>For the first way, use the fact that this is a <strong>conditional<\/strong> and changes the sample space. The graph illustrates the new sample space. You already know the baby smiled more than eight seconds. Write a new\u00a0<em>f<\/em>(<em>x<\/em>):[latex]{f{{({x})}}}=\\frac{{1}}{{{23}-{8}}}=\\frac{{1}}{{15}}[\/latex]for 8 &lt; <em>x<\/em> &lt; 23\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/odic-jg5e627i#fixme#fixme#fixme\" alt=\"f(X)=1\/15 graph displaying a boxed region consisting of a horizontal line extending to the right from point 1\/15 on the y-axis, a vertical upward line from points 8 and 23 on the x-axis, and the x-axis. A shaded region from points 12-23 occurs within this area.\" \/><\/li>\r\n<\/ol>\r\n<ol>\r\n \t<li>For the second way, use the conditional formula (shown below) with the original distribution <em>X<\/em> ~ <em>U<\/em> (0, 23): For this problem,\u00a0<em>A<\/em> is (<em>x<\/em> &gt; 12) and <em>B<\/em> is (<em>x<\/em> &gt; 8).\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/9puf-qk5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/23. A horiztonal line ranges from the point (0, 1\/23) to the point (23, 1\/23). A vertical line extends from the x-axis to the end of the line at point (23, 1\/23) creating a rectangle. Vertical lines extend from the horizontal axis to the graph at x = 8 and x = 12.\" \/><\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Try It<\/h3>\r\nA distribution is given as [latex]X{\\sim}U(0,20)[\/latex]. What is [latex]P(2&lt;x&lt;18)[\/latex]? Find the 90th percentile.\r\n\r\n[reveal-answer q=\"306492\"]Show Solution[\/reveal-answer]\r\n[hidden-answer a=\"306492\"]\r\n\r\n[latex]P(2&lt;x&lt;18)=0.8[\/latex]; 90th percentile[latex]=18[\/latex]\r\n\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Example<\/h3>\r\nThe amount of time, in minutes, that a person must wait for a bus is uniformly distributed between 0 and 15 minutes, inclusive.\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>What is the probability that a person waits fewer than 12.5 minutes?<\/li>\r\n \t<li>On the average, how long must a person wait? Find the mean, <em>\u03bc<\/em>, and the standard deviation, <em>\u03c3<\/em>.<\/li>\r\n \t<li>Ninety percent of the time, the time a person must wait falls below what value? This asks for the <strong>90th percentile<\/strong>.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"431436\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"431436\"]\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Let <em>X<\/em> = the number of minutes a person must wait for a bus. <em>a<\/em> = 0 and <em>b<\/em> = 15. <em>X<\/em>~ <em>U<\/em>(0, 15). Write the probability density function. [latex]{f{{({x})}}}=\\frac{{1}}{{{15}-{0}}}=\\frac{{1}}{{15}}[\/latex] for 0 \u2264<em>x<\/em> \u2264 15. Find\u00a0<em>P<\/em> (<em>x<\/em> &lt; 12.5). Draw a graph.\r\n[latex]{P}{({x}{&lt;}{k})}={(\\text{base})}{(\\text{height})}={({12.5}-{0})}{(\\frac{{1}}{{15}})}={0.8333}[\/latex]\r\nThe probability a person waits less than 12.5 minutes is 0.8333.\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/jt07-1p5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/15. A horiztonal line ranges from the point (0, 1\/15) to the point (15, 1\/15). A vertical line extends from the x-axis to the end of the line at point (15, 1\/15) creating a rectangle. A region is shaded inside the rectangle from x = 0 to x = 12.5.\" \/><\/li>\r\n \t<li>[latex]{\\mu}=\\frac{{{a}+{b}}}{{2}}=\\frac{{{15}+{0}}}{{2}}={7.5}[\/latex]. On the average, a person must wait 7.5 minutes.[latex]{\\sigma}=\\sqrt{{\\frac{{{({b}-{a})}^{{2}}}}{{12}}}}=\\sqrt{{\\frac{{{({15}-{0})}^{{2}}}}{{12}}}}={4.3}[\/latex]. The standard deviation is 4.3 minutes.<\/li>\r\n \t<li>Find the 90th percentile. Draw a graph. Let <em>k<\/em> = the 90th percentile.\r\n[latex]{P}(x&lt;k)=(\\text{base})(\\text{height})=(k-0)(\\frac{1}{15})[\/latex]\r\n[latex]{0.90}=(k)(\\frac{1}{15})[\/latex]\r\n[latex]k=(0.90)(15)=13.5[\/latex]\r\n<em>k<\/em> is sometimes called a critical value. The 90th percentile is 13.5 minutes. Ninety percent of the time, a person must wait at most 13.5 minutes.\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/zovz-0t5e627i#fixme#fixme#fixme\" alt=\"f(X)=1\/15 graph displaying a boxed region consisting of a horizontal line extending to the right from point 1\/15 on the y-axis, a vertical upward line from an arbitrary point on the x-axis, and the x and y-axes. A shaded region from points 0-k occurs within this area. The area of this probability region is equal to 0.90.\" \/><\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Try It<\/h3>\r\nThe total duration of baseball games in the major league in the 2011 season is uniformly distributed between 447 hours and 521 hours inclusive.\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Find <em>a<\/em> and <em>b<\/em> and describe what they represent.<\/li>\r\n \t<li>Write the distribution.<\/li>\r\n \t<li>Find the mean and the standard deviation.<\/li>\r\n \t<li>What is the probability that the duration of games for a team for the 2011 season is between 480 and 500 hours?<\/li>\r\n \t<li>What is the 65th percentile for the duration of games for a team for the 2011 season?<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"741605\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"741605\"]\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li><em>a<\/em> is 447, and <em>b<\/em> is 521. <em>a<\/em> is the minimum duration of games for a team for the 2011 season, and <em>b<\/em> is the maximum duration of games for a team for the 2011 season.<\/li>\r\n \t<li><em>X<\/em> ~ <em>U<\/em> (447, 521).<\/li>\r\n \t<li><em>\u03bc<\/em> = 484, and <em>\u03c3<\/em> = 21.36\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/ld91-516e627i#fixme#fixme#fixme\" alt=\"\" \/><\/li>\r\n \t<li>[latex]P(480&lt;x&lt; 500) = 0.2703[\/latex]<\/li>\r\n \t<li>65th percentile is 495.1 hours.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox examples\">\r\n<h3>Recall: Fraction Divide by a Fraction<\/h3>\r\n<ol>\r\n \t<li>Find the reciprocal of the fraction that follows the division symbol.\r\n<ul>\r\n \t<li>The reciprocal of [latex]\\frac{1}{2}[\/latex] is [latex]\\frac{2}{1}[\/latex].<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>Multiply the first fraction (the one before the division symbol) by the reciprocal of the second fraction (the one after the division symbol).\r\n<ul>\r\n \t<li>[latex]\\frac{3}{4} \\div \\frac{1}{2} = \\frac{3}{4} \\times \\frac{2}{1} = \\frac{3 \\cdot 2}{4 \\cdot 1} = \\frac{6}{4} = 1 \\frac{1}{2}[\/latex].<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n&nbsp;\r\n\r\n<hr \/>\r\n\r\n<div class=\"textbox exercises\">\r\n<h3>Example<\/h3>\r\nSuppose the time it takes a nine-year-old child to eat a donut is between 0.5 and 4 minutes, inclusive. Let\u00a0<em>X<\/em> = the time, in minutes, it takes a nine-year-old to eat a donut. Then <em>X<\/em>~ <em>U<\/em> (0.5, 4).\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>The probability that a randomly selected nine-year-old eats a donut in at least two minutes is _______.<\/li>\r\n \t<li>Find the probability that a different nine-year-old eats a donut in more than two minutes given that the child has already been eating the donut for more than 1.5 minutes.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"782990\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"782990\"]\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>0.5714<\/li>\r\n \t<li>This question has a conditional probability. You are asked to find the probability that a nine-year-old eats a donut in more than two minutes given that the child has already been eating the donut for more than 1.5 minutes. Solve the problem two different ways (see previous Example). You must reduce the sample space.\r\n<ol>\r\n \t<li>First way: Since you know the child has already been eating the donut for more than 1.5 minutes, you are no longer starting at <em>a<\/em> = 0.5 minutes. Your starting point is 1.5 minutes. Write a new\u00a0<em>f<\/em>(<em>x<\/em>):[latex]{f{{({x})}}}=\\frac{{1}}{{{4}-{1.5}}}={25} \\text{ for } {1.5}\\leq{x}\\leq{4}[\/latex]. Find\u00a0<em>P<\/em>(<em>x<\/em> &gt; 2|<em>x<\/em> &gt; 1.5). Draw a graph.\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/75d7-b56e627i#fixme#fixme#fixme\" alt=\"f(X)=2\/5 graph displaying a boxed region consisting of a horizontal line extending to the right from point 2\/5 on the y-axis, a vertical upward line from points 1.5 and 4 on the x-axis, and the x-axis. A shaded region from points 2-4 occurs within this area.\" \/>[latex]{P}{({x}{&gt;}{2}|{x}{&gt;}{1.5})}={(\\text{base})}{(\\text{new height})}={({4}-{2})}{(\\frac{{2}}{{5}})}=\\frac{{4}}{{5}}[\/latex]\r\nThe probability that a nine-year old child eats a donut in more than two minutes, given that the child has already been eating the donut for more than 1.5 minutes, is [latex]\\frac{{4}}{{5}}[\/latex].<\/li>\r\n \t<li>Second way: Draw the original graph for <em>X<\/em> ~ <em>U<\/em> (0.5, 4). Use the conditional formula[latex]{P}{({x}{&gt;}{2}{mid}{x}{&gt;}{1.5})}=\\frac{{{P}{({x}{&gt;}{2} \\text{ AND } {x}{&gt;}{1.5})}}}{{{P}{({x}{&gt;}{1.5})}}}=\\frac{{{P}{({x}{&gt;}{2})}}}{{{P}{({x}{&gt;}{1.5})}}}=\\frac{{\\frac{{2}}{{3.5}}}}{{\\frac{{2.5}}{{3.5}}}}={0.8}=\\frac{{4}}{{5}}[\/latex]<\/li>\r\n<\/ol>\r\n<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div><\/div>\r\n&nbsp;\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Try It<\/h3>\r\nSuppose the time it takes a student to finish a quiz is uniformly distributed between six and 15 minutes, inclusive. Let\u00a0<em>X<\/em> = the time, in minutes, it takes a student to finish a quiz. Then <em>X<\/em> ~ <em>U<\/em> (6, 15).\r\n\r\nFind the probability that a randomly selected student needs at least eight minutes to complete the quiz. Then find the probability that a different student needs at least eight minutes to finish the quiz given that she has already taken more than seven minutes.\r\n\r\n[reveal-answer q=\"881073\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"881073\"]\r\n\r\n[latex]P(x&gt;8) = 0.7778[\/latex]\r\n\r\n[latex]P(x&gt;8 | x&gt;7) = 0.875[\/latex]\r\n\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Example<\/h3>\r\nAce Heating and Air Conditioning Service finds that the amount of time a repairman needs to fix a furnace is uniformly distributed between 1.5 and four hours. Let\u00a0<em>x<\/em> = the time needed to fix a furnace. Then <em>x<\/em> ~ <em>U<\/em> (1.5, 4).\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Find the probability that a randomly selected furnace repair requires more than two hours.<\/li>\r\n \t<li>Find the probability that a randomly selected furnace repair requires less than three hours.<\/li>\r\n \t<li>Find the 30th percentile of furnace repair times.<\/li>\r\n \t<li>The longest 25% of furnace repair times take at least how long? (In other words: find the minimum time for the longest 25% of repair times.) What percentile does this represent?<\/li>\r\n \t<li>Find the mean and standard deviation.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"237308\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"237308\"]\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>To find\u00a0<em>f<\/em>(<em>x<\/em>): [latex]{f{{({x})}}}=\\frac{{1}}{{{4}-{1.5}}}={12.5} \\text{ so } {f{{({x})}}}={0.4}[\/latex]\r\n<em>P<\/em>(<em>x<\/em> &gt; 2) = (base)(height) = (4 \u2013 2)(0.4) = 0.8\r\n<img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/42bq-596e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 2 to x = 4.\" \/>\r\nUniform Distribution between 1.5 and four with shaded area between two and four representing the probability that the repair time\r\n<em>x<\/em> is greater than two<\/li>\r\n \t<li><em>P<\/em>(<em>x<\/em> &lt; 3) = (base)(height) = (3 \u2013 1.5)(0.4) = 0.6. The graph of the rectangle showing the entire distribution would remain the same. However the graph should be shaded between\r\n<em>x<\/em> = 1.5 and <em>x<\/em> = 3. Note that the shaded area starts at <em>x<\/em> = 1.5 rather than at <em>x<\/em> = 0; since <em>X<\/em> ~ <em>U<\/em> (1.5, 4), <em>x <\/em>cannot be less than 1.5.<\/li>\r\n \t<li><img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/3x2o-ad6e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 1.5 to x = 3.\" \/>\r\nUniform Distribution between 1.5 and four with shaded area between 1.5 and three representing the probability that the repair time\u00a0<em>x<\/em> is less than three<\/li>\r\n \t<li><img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/1yjh-4h6e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 1.5 to x = k. The shaded area represents P(x &lt; k) = 0.3.\" \/>\r\nUniform Distribution between 1.5 and 4 with an area of 0.30 shaded to the left, representing the shortest 30% of repair times.\r\n[latex]P(x&lt;k) = 0.30[\/latex]\r\n[latex]P(x&lt;k)[\/latex] = (base)(height) = [latex](<em>k<\/em> \u2013 1.5)(0.4)0.3 = (<em>k<\/em> \u2013 1.5) (0.4)[\/latex]; Solve to find <em>k<\/em>: 0.75 =\u00a0<em>k<\/em> \u2013 1.5, obtained by dividing both sides by 0.4\r\n<em>k<\/em> = 2.25 , obtained by adding 1.5 to both sides. The 30th percentile of repair times is 2.25 hours. 30% of repair times are 2.5 hours or less.<\/li>\r\n \t<li><img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/h8bp-3l6e627i#fixme#fixme#fixme\" alt=\"\" \/>\r\nUniform Distribution between 1.5 and 4 with an area of 0.25 shaded to the right representing the longest 25% of repair times. [latex]P(x&gt;k)[\/latex] = 0.25\r\n[latex]P(x&gt;k)[\/latex] = (base)(height) = [latex](4 \u2013 k)(0.4)0.25 = (4\u2013k)(0.4)[\/latex]; Solve for <em>k<\/em>: 0.625 = 4 \u2212\u00a0<em>k<\/em>, obtained by dividing both sides by 0.4<\/li>\r\n<\/ol>\r\n\u22123.375 = \u2212\u00a0<em>k<\/em>, obtained by subtracting four from both sides: <em>k<\/em> = 3.375\r\n\r\nThe longest 25% of furnace repairs take at least 3.375 hours (3.375 hours or longer).\r\n\r\n<strong>Note:<\/strong> Since 25% of repair times are 3.375 hours or longer, that means that 75% of repair times are 3.375 hours or less. 3.375 hours is the <strong>75th percentile<\/strong> of furnace repair times.\r\n<ul>\r\n \t<li>[latex]{\\mu}={\\frac{a+b}{2}}\\text{ and }{\\sigma}=\\sqrt{\\frac{(b-a)^2}{12}}[\/latex]\r\n[latex]{\\mu}=\\frac{1.5+4}{2}=2.75\\text{ hours and }{\\sigma}=\\sqrt{\\frac{(4-1.5)^2}{12}}= 0.7217 \\text{ hours}[\/latex]<\/li>\r\n<\/ul>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Try It<\/h3>\r\nThe amount of time a service technician needs to change the oil in a car is uniformly distributed between 11 and 21 minutes. Let\u00a0<em>X<\/em> = the time needed to change the oil on a car.\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Write the random variable <em>X<\/em> in words. <em>X<\/em> = __________________.<\/li>\r\n \t<li>Write the distribution.<\/li>\r\n \t<li>Graph the distribution.<\/li>\r\n \t<li>Find [latex]P(x&gt;19)[\/latex].<\/li>\r\n \t<li>Find the 50th percentile.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"962497\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"962497\"]\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>Let <em>X<\/em> = the time needed to change the oil in a car.<\/li>\r\n \t<li><em>X<\/em> ~ <em>U<\/em> (11, 21).<\/li>\r\n \t<li><img src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/ld91-516e627i#fixme#fixme#fixme\" alt=\"This graph shows a uniform distribution. The horizontal axis ranges from 405 to 525. The distribution is modeled by a rectangle extending from x = 447 to x = 521.\" \/><\/li>\r\n \t<li>[latex]P(x&gt;19) = 0.2[\/latex]<\/li>\r\n \t<li>the 50th percentile is 16 minutes.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Calculate the mean and standard deviation for a uniform distribution<\/li>\n<li>Calculate probabilities, including conditional probabilities, for a uniform distribution<\/li>\n<li>Calculate percentiles for a uniform distribution<\/li>\n<\/ul>\n<\/div>\n<p>The <strong>uniform distribution<\/strong> is a continuous probability distribution and is concerned with events that are equally likely to occur. When working out problems that have a uniform distribution, be careful to note if the data is inclusive or exclusive.<\/p>\n<div class=\"textbox exercises\">\n<h3>Example<\/h3>\n<p>The data in the table below are 55 smiling times, in seconds, of an eight-week-old baby.<\/p>\n<table>\n<tbody>\n<tr>\n<td scope=\"row\">10.4<\/td>\n<td>19.6<\/td>\n<td>18.8<\/td>\n<td>13.9<\/td>\n<td>17.8<\/td>\n<td>16.8<\/td>\n<td>21.6<\/td>\n<td>17.9<\/td>\n<td>12.5<\/td>\n<td>11.1<\/td>\n<td>4.9<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">12.8<\/td>\n<td>14.8<\/td>\n<td>22.8<\/td>\n<td>20.0<\/td>\n<td>15.9<\/td>\n<td>16.3<\/td>\n<td>13.4<\/td>\n<td>17.1<\/td>\n<td>14.5<\/td>\n<td>19.0<\/td>\n<td>22.8<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">1.3<\/td>\n<td>0.7<\/td>\n<td>8.9<\/td>\n<td>11.9<\/td>\n<td>10.9<\/td>\n<td>7.3<\/td>\n<td>5.9<\/td>\n<td>3.7<\/td>\n<td>17.9<\/td>\n<td>19.2<\/td>\n<td>9.8<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">5.8<\/td>\n<td>6.9<\/td>\n<td>2.6<\/td>\n<td>5.8<\/td>\n<td>21.7<\/td>\n<td>11.8<\/td>\n<td>3.4<\/td>\n<td>2.1<\/td>\n<td>4.5<\/td>\n<td>6.3<\/td>\n<td>10.7<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">8.9<\/td>\n<td>9.4<\/td>\n<td>9.4<\/td>\n<td>7.6<\/td>\n<td>10.0<\/td>\n<td>3.3<\/td>\n<td>6.7<\/td>\n<td>7.8<\/td>\n<td>11.6<\/td>\n<td>13.8<\/td>\n<td>18.6<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>The sample mean[latex]=11.49[\/latex] and the sample standard deviation[latex]=6.23[\/latex].<\/p>\n<p>We will assume that the smiling times, in seconds, follow a uniform distribution between zero and 23 seconds, inclusive. This means that any smiling time from zero to and including 23 seconds is <strong>equally likely<\/strong>. The histogram that could be constructed from the sample is an empirical distribution that closely matches the theoretical uniform distribution.<\/p>\n<p>Let [latex]X=[\/latex] length, in seconds, of an eight-week-old baby&#8217;s smile.<\/p>\n<p>The notation for the uniform distribution is [latex]X{\\sim}U(a,b)[\/latex] where [latex]a=[\/latex] the lowest value of [latex]x[\/latex]\u00a0and\u00a0[latex]b=[\/latex] the highest value of [latex]x[\/latex].<\/p>\n<p>The probability density function is [latex]{f{{({x})}}}=\\frac{{1}}{{{b}-{a}}}[\/latex] for [latex]a{\\leq}x{\\leq}b[\/latex].<\/p>\n<p>For this example, [latex]X{\\sim}U(0,23)[\/latex] and [latex]{f{{({x})}}}=\\frac{{1}}{{{23}-{0}}}[\/latex] for [latex]0{\\leq}X{\\leq}23[\/latex].<\/p>\n<p>Formulas for the theoretical mean and standard deviation are [latex]{\\mu}=\\frac{{{a}+{b}}}{{2}}{\\quad\\text{and}\\quad}{\\sigma}=\\sqrt{{\\frac{{{({b}-{a})}^{{2}}}}{{12}}}}[\/latex]<\/p>\n<p>For this problem, the theoretical mean and standard deviation are [latex]{\\mu}=\\frac{{{0}+{23}}}{{2}}={11.50} \\text{ seconds}{\\quad\\text{and}\\quad}{\\sigma}=\\sqrt{{\\frac{{{({23}-{0})}^{{2}}}}{{12}}}}={6.64} \\text{ seconds}[\/latex]<\/p>\n<p>Notice that the theoretical mean and standard deviation are close to the sample mean and standard deviation in this example.<\/p>\n<\/div>\n<hr \/>\n<div class=\"textbox key-takeaways\">\n<h3>Try It<\/h3>\n<p>The data that follow are the number of passengers on 35 different charter fishing boats. The sample mean [latex]=7.9[\/latex] and the sample standard deviation [latex]=4.33[\/latex]. The data follow a uniform distribution where all values between and including zero and 14 are equally likely. State the values of\u00a0<em>a<\/em> and <em>b<\/em>. Write the distribution in proper notation, and calculate the theoretical mean and standard deviation.<\/p>\n<table>\n<tbody>\n<tr>\n<td scope=\"row\">1<\/td>\n<td>12<\/td>\n<td>4<\/td>\n<td>10<\/td>\n<td>4<\/td>\n<td>14<\/td>\n<td>11<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">7<\/td>\n<td>11<\/td>\n<td>4<\/td>\n<td>13<\/td>\n<td>2<\/td>\n<td>4<\/td>\n<td>6<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">3<\/td>\n<td>10<\/td>\n<td>0<\/td>\n<td>12<\/td>\n<td>6<\/td>\n<td>9<\/td>\n<td>10<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">5<\/td>\n<td>13<\/td>\n<td>4<\/td>\n<td>10<\/td>\n<td>14<\/td>\n<td>12<\/td>\n<td>11<\/td>\n<\/tr>\n<tr>\n<td scope=\"row\">6<\/td>\n<td>10<\/td>\n<td>11<\/td>\n<td>0<\/td>\n<td>11<\/td>\n<td>13<\/td>\n<td>2<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q810841\">Show Solution<\/span><\/p>\n<div id=\"q810841\" class=\"hidden-answer\" style=\"display: none\">\n<p><em>a<\/em> is zero;<em> b<\/em> is 14; [latex]X{\\sim}U(0,14)[\/latex]; [latex]\\mu=7[\/latex] passengers; [latex]\\sigma=4.04[\/latex] passengers<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Example<\/h3>\n<ol>\n<li>Refer to the above example about the eight-week-old baby. What is the probability that a randomly chosen eight-week-old baby smiles between two and 18 seconds?<\/li>\n<li>Find the 90th percentile for an eight-week-old baby&#8217;s smiling time.<\/li>\n<li>Find the probability that a random eight-week-old baby smiles more than 12 seconds knowing that the baby smiles more than eight seconds.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q96316\">Show Solution<\/span><\/p>\n<div id=\"q96316\" class=\"hidden-answer\" style=\"display: none\">\n<ol>\n<li>Find <em>P<\/em>(2 &lt; <em>x<\/em> &lt; 18).<br \/>\n[latex]{P}{({2}{<}{x}{<}{18})}={(\\text{base})}{(\\text{height})}={({18}-{2})}{(\\frac{{1}}{{23}})}={(\\frac{{16}}{{23}})}.[\/latex]<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/80tq-575e627i#fixme#fixme#fixme\" alt=\"This graph shows a uniform distribution. The horizontal axis ranges from 0 to 15. The distribution is modeled by a rectangle extending from x = 0 to x = 15. A region from x = 2 to x = 18 is shaded inside the rectangle.\" \/><\/li>\n<li>Ninety percent of the smiling times fall below the 90th percentile, <em>k<\/em>, so <em>P<\/em>(<em>x<\/em> &lt;<em>k<\/em>) = 0.90<br \/>\n[latex]P(x<k)=0.90[\/latex]\n[latex](\\text{base})(\\text{height})=0.90[\/latex]\n[latex](k-0)(\\frac{1}{23})=0.90[\/latex]\n[latex]k=(23)(0.90)=20.7[\/latex]\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/5ivo-nc5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/15. A horiztonal line ranges from the point (0, 1\/15) to the point (15, 1\/15). A vertical line extends from the x-axis to the end of the line at point (15, 1\/15) creating a rectangle. A region is shaded inside the rectangle from x = 0 to x = k. The shaded area represents P(x &lt; k) = 0.90.\" \/><\/li>\n<li>This probability question is a <strong>conditional<\/strong>. You are asked to find the probability that an eight-week-old baby smiles more than 12 seconds when you already know the baby has smiled for more than eight seconds. Find\u00a0<em>P<\/em>(<em>x<\/em> &gt; 12|<em>x<\/em> &gt; 8) There are two ways to do the problem.\n<ol>\n<li>For the first way, use the fact that this is a <strong>conditional<\/strong> and changes the sample space. The graph illustrates the new sample space. You already know the baby smiled more than eight seconds. Write a new\u00a0<em>f<\/em>(<em>x<\/em>):[latex]{f{{({x})}}}=\\frac{{1}}{{{23}-{8}}}=\\frac{{1}}{{15}}[\/latex]for 8 &lt; <em>x<\/em> &lt; 23<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/odic-jg5e627i#fixme#fixme#fixme\" alt=\"f(X)=1\/15 graph displaying a boxed region consisting of a horizontal line extending to the right from point 1\/15 on the y-axis, a vertical upward line from points 8 and 23 on the x-axis, and the x-axis. A shaded region from points 12-23 occurs within this area.\" \/><\/li>\n<\/ol>\n<ol>\n<li>For the second way, use the conditional formula (shown below) with the original distribution <em>X<\/em> ~ <em>U<\/em> (0, 23): For this problem,\u00a0<em>A<\/em> is (<em>x<\/em> &gt; 12) and <em>B<\/em> is (<em>x<\/em> &gt; 8).<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/9puf-qk5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/23. A horiztonal line ranges from the point (0, 1\/23) to the point (23, 1\/23). A vertical line extends from the x-axis to the end of the line at point (23, 1\/23) creating a rectangle. Vertical lines extend from the horizontal axis to the graph at x = 8 and x = 12.\" \/><\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Try It<\/h3>\n<p>A distribution is given as [latex]X{\\sim}U(0,20)[\/latex]. What is [latex]P(2<x<18)[\/latex]? Find the 90th percentile.\n\n\n\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q306492\">Show Solution<\/span><\/p>\n<div id=\"q306492\" class=\"hidden-answer\" style=\"display: none\">\n<p>[latex]P(2<x<18)=0.8[\/latex]; 90th percentile[latex]=18[\/latex]\n\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Example<\/h3>\n<p>The amount of time, in minutes, that a person must wait for a bus is uniformly distributed between 0 and 15 minutes, inclusive.<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>What is the probability that a person waits fewer than 12.5 minutes?<\/li>\n<li>On the average, how long must a person wait? Find the mean, <em>\u03bc<\/em>, and the standard deviation, <em>\u03c3<\/em>.<\/li>\n<li>Ninety percent of the time, the time a person must wait falls below what value? This asks for the <strong>90th percentile<\/strong>.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q431436\">Show Answer<\/span><\/p>\n<div id=\"q431436\" class=\"hidden-answer\" style=\"display: none\">\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Let <em>X<\/em> = the number of minutes a person must wait for a bus. <em>a<\/em> = 0 and <em>b<\/em> = 15. <em>X<\/em>~ <em>U<\/em>(0, 15). Write the probability density function. [latex]{f{{({x})}}}=\\frac{{1}}{{{15}-{0}}}=\\frac{{1}}{{15}}[\/latex] for 0 \u2264<em>x<\/em> \u2264 15. Find\u00a0<em>P<\/em> (<em>x<\/em> &lt; 12.5). Draw a graph.<br \/>\n[latex]{P}{({x}{<}{k})}={(\\text{base})}{(\\text{height})}={({12.5}-{0})}{(\\frac{{1}}{{15}})}={0.8333}[\/latex]\nThe probability a person waits less than 12.5 minutes is 0.8333.\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/jt07-1p5e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 1\/15. A horiztonal line ranges from the point (0, 1\/15) to the point (15, 1\/15). A vertical line extends from the x-axis to the end of the line at point (15, 1\/15) creating a rectangle. A region is shaded inside the rectangle from x = 0 to x = 12.5.\" \/><\/li>\n<li>[latex]{\\mu}=\\frac{{{a}+{b}}}{{2}}=\\frac{{{15}+{0}}}{{2}}={7.5}[\/latex]. On the average, a person must wait 7.5 minutes.[latex]{\\sigma}=\\sqrt{{\\frac{{{({b}-{a})}^{{2}}}}{{12}}}}=\\sqrt{{\\frac{{{({15}-{0})}^{{2}}}}{{12}}}}={4.3}[\/latex]. The standard deviation is 4.3 minutes.<\/li>\n<li>Find the 90th percentile. Draw a graph. Let <em>k<\/em> = the 90th percentile.<br \/>\n[latex]{P}(x<k)=(\\text{base})(\\text{height})=(k-0)(\\frac{1}{15})[\/latex]\n[latex]{0.90}=(k)(\\frac{1}{15})[\/latex]\n[latex]k=(0.90)(15)=13.5[\/latex]\n<em>k<\/em> is sometimes called a critical value. The 90th percentile is 13.5 minutes. Ninety percent of the time, a person must wait at most 13.5 minutes.<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/zovz-0t5e627i#fixme#fixme#fixme\" alt=\"f(X)=1\/15 graph displaying a boxed region consisting of a horizontal line extending to the right from point 1\/15 on the y-axis, a vertical upward line from an arbitrary point on the x-axis, and the x and y-axes. A shaded region from points 0-k occurs within this area. The area of this probability region is equal to 0.90.\" \/><\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Try It<\/h3>\n<p>The total duration of baseball games in the major league in the 2011 season is uniformly distributed between 447 hours and 521 hours inclusive.<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Find <em>a<\/em> and <em>b<\/em> and describe what they represent.<\/li>\n<li>Write the distribution.<\/li>\n<li>Find the mean and the standard deviation.<\/li>\n<li>What is the probability that the duration of games for a team for the 2011 season is between 480 and 500 hours?<\/li>\n<li>What is the 65th percentile for the duration of games for a team for the 2011 season?<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q741605\">Show Answer<\/span><\/p>\n<div id=\"q741605\" class=\"hidden-answer\" style=\"display: none\">\n<ol style=\"list-style-type: lower-alpha;\">\n<li><em>a<\/em> is 447, and <em>b<\/em> is 521. <em>a<\/em> is the minimum duration of games for a team for the 2011 season, and <em>b<\/em> is the maximum duration of games for a team for the 2011 season.<\/li>\n<li><em>X<\/em> ~ <em>U<\/em> (447, 521).<\/li>\n<li><em>\u03bc<\/em> = 484, and <em>\u03c3<\/em> = 21.36<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/ld91-516e627i#fixme#fixme#fixme\" alt=\"\" \/><\/li>\n<li>[latex]P(480<x< 500) = 0.2703[\/latex]<\/li>\n<li>65th percentile is 495.1 hours.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox examples\">\n<h3>Recall: Fraction Divide by a Fraction<\/h3>\n<ol>\n<li>Find the reciprocal of the fraction that follows the division symbol.\n<ul>\n<li>The reciprocal of [latex]\\frac{1}{2}[\/latex] is [latex]\\frac{2}{1}[\/latex].<\/li>\n<\/ul>\n<\/li>\n<li>Multiply the first fraction (the one before the division symbol) by the reciprocal of the second fraction (the one after the division symbol).\n<ul>\n<li>[latex]\\frac{3}{4} \\div \\frac{1}{2} = \\frac{3}{4} \\times \\frac{2}{1} = \\frac{3 \\cdot 2}{4 \\cdot 1} = \\frac{6}{4} = 1 \\frac{1}{2}[\/latex].<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<\/div>\n<p>&nbsp;<\/p>\n<hr \/>\n<div class=\"textbox exercises\">\n<h3>Example<\/h3>\n<p>Suppose the time it takes a nine-year-old child to eat a donut is between 0.5 and 4 minutes, inclusive. Let\u00a0<em>X<\/em> = the time, in minutes, it takes a nine-year-old to eat a donut. Then <em>X<\/em>~ <em>U<\/em> (0.5, 4).<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>The probability that a randomly selected nine-year-old eats a donut in at least two minutes is _______.<\/li>\n<li>Find the probability that a different nine-year-old eats a donut in more than two minutes given that the child has already been eating the donut for more than 1.5 minutes.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q782990\">Show Answer<\/span><\/p>\n<div id=\"q782990\" class=\"hidden-answer\" style=\"display: none\">\n<ol style=\"list-style-type: lower-alpha;\">\n<li>0.5714<\/li>\n<li>This question has a conditional probability. You are asked to find the probability that a nine-year-old eats a donut in more than two minutes given that the child has already been eating the donut for more than 1.5 minutes. Solve the problem two different ways (see previous Example). You must reduce the sample space.\n<ol>\n<li>First way: Since you know the child has already been eating the donut for more than 1.5 minutes, you are no longer starting at <em>a<\/em> = 0.5 minutes. Your starting point is 1.5 minutes. Write a new\u00a0<em>f<\/em>(<em>x<\/em>):[latex]{f{{({x})}}}=\\frac{{1}}{{{4}-{1.5}}}={25} \\text{ for } {1.5}\\leq{x}\\leq{4}[\/latex]. Find\u00a0<em>P<\/em>(<em>x<\/em> &gt; 2|<em>x<\/em> &gt; 1.5). Draw a graph.<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/75d7-b56e627i#fixme#fixme#fixme\" alt=\"f(X)=2\/5 graph displaying a boxed region consisting of a horizontal line extending to the right from point 2\/5 on the y-axis, a vertical upward line from points 1.5 and 4 on the x-axis, and the x-axis. A shaded region from points 2-4 occurs within this area.\" \/>[latex]{P}{({x}{>}{2}|{x}{>}{1.5})}={(\\text{base})}{(\\text{new height})}={({4}-{2})}{(\\frac{{2}}{{5}})}=\\frac{{4}}{{5}}[\/latex]<br \/>\nThe probability that a nine-year old child eats a donut in more than two minutes, given that the child has already been eating the donut for more than 1.5 minutes, is [latex]\\frac{{4}}{{5}}[\/latex].<\/li>\n<li>Second way: Draw the original graph for <em>X<\/em> ~ <em>U<\/em> (0.5, 4). Use the conditional formula[latex]{P}{({x}{>}{2}{mid}{x}{>}{1.5})}=\\frac{{{P}{({x}{>}{2} \\text{ AND } {x}{>}{1.5})}}}{{{P}{({x}{>}{1.5})}}}=\\frac{{{P}{({x}{>}{2})}}}{{{P}{({x}{>}{1.5})}}}=\\frac{{\\frac{{2}}{{3.5}}}}{{\\frac{{2.5}}{{3.5}}}}={0.8}=\\frac{{4}}{{5}}[\/latex]<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<div><\/div>\n<p>&nbsp;<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Try It<\/h3>\n<p>Suppose the time it takes a student to finish a quiz is uniformly distributed between six and 15 minutes, inclusive. Let\u00a0<em>X<\/em> = the time, in minutes, it takes a student to finish a quiz. Then <em>X<\/em> ~ <em>U<\/em> (6, 15).<\/p>\n<p>Find the probability that a randomly selected student needs at least eight minutes to complete the quiz. Then find the probability that a different student needs at least eight minutes to finish the quiz given that she has already taken more than seven minutes.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q881073\">Show Answer<\/span><\/p>\n<div id=\"q881073\" class=\"hidden-answer\" style=\"display: none\">\n<p>[latex]P(x>8) = 0.7778[\/latex]<\/p>\n<p>[latex]P(x>8 | x>7) = 0.875[\/latex]<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Example<\/h3>\n<p>Ace Heating and Air Conditioning Service finds that the amount of time a repairman needs to fix a furnace is uniformly distributed between 1.5 and four hours. Let\u00a0<em>x<\/em> = the time needed to fix a furnace. Then <em>x<\/em> ~ <em>U<\/em> (1.5, 4).<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Find the probability that a randomly selected furnace repair requires more than two hours.<\/li>\n<li>Find the probability that a randomly selected furnace repair requires less than three hours.<\/li>\n<li>Find the 30th percentile of furnace repair times.<\/li>\n<li>The longest 25% of furnace repair times take at least how long? (In other words: find the minimum time for the longest 25% of repair times.) What percentile does this represent?<\/li>\n<li>Find the mean and standard deviation.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q237308\">Show Answer<\/span><\/p>\n<div id=\"q237308\" class=\"hidden-answer\" style=\"display: none\">\n<ol style=\"list-style-type: lower-alpha;\">\n<li>To find\u00a0<em>f<\/em>(<em>x<\/em>): [latex]{f{{({x})}}}=\\frac{{1}}{{{4}-{1.5}}}={12.5} \\text{ so } {f{{({x})}}}={0.4}[\/latex]<br \/>\n<em>P<\/em>(<em>x<\/em> &gt; 2) = (base)(height) = (4 \u2013 2)(0.4) = 0.8<br \/>\n<img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/42bq-596e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 2 to x = 4.\" \/><br \/>\nUniform Distribution between 1.5 and four with shaded area between two and four representing the probability that the repair time<br \/>\n<em>x<\/em> is greater than two<\/li>\n<li><em>P<\/em>(<em>x<\/em> &lt; 3) = (base)(height) = (3 \u2013 1.5)(0.4) = 0.6. The graph of the rectangle showing the entire distribution would remain the same. However the graph should be shaded between<br \/>\n<em>x<\/em> = 1.5 and <em>x<\/em> = 3. Note that the shaded area starts at <em>x<\/em> = 1.5 rather than at <em>x<\/em> = 0; since <em>X<\/em> ~ <em>U<\/em> (1.5, 4), <em>x <\/em>cannot be less than 1.5.<\/li>\n<li><img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/3x2o-ad6e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 1.5 to x = 3.\" \/><br \/>\nUniform Distribution between 1.5 and four with shaded area between 1.5 and three representing the probability that the repair time\u00a0<em>x<\/em> is less than three<\/li>\n<li><img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/1yjh-4h6e627i#fixme#fixme#fixme\" alt=\"This shows the graph of the function f(x) = 0.4. A horiztonal line ranges from the point (1.5, 0.4) to the point (4, 0.4). Vertical lines extend from the x-axis to the graph at x = 1.5 and x = 4 creating a rectangle. A region is shaded inside the rectangle from x = 1.5 to x = k. The shaded area represents P(x &lt; k) = 0.3.\" \/><br \/>\nUniform Distribution between 1.5 and 4 with an area of 0.30 shaded to the left, representing the shortest 30% of repair times.<br \/>\n[latex]P(x<k) = 0.30[\/latex]\n[latex]P(x<k)[\/latex] = (base)(height) = [latex](<em>k<\/em> \u2013 1.5)(0.4)0.3 = (<em>k<\/em> \u2013 1.5) (0.4)[\/latex]; Solve to find <em>k<\/em>: 0.75 =\u00a0<em>k<\/em> \u2013 1.5, obtained by dividing both sides by 0.4<br \/>\n<em>k<\/em> = 2.25 , obtained by adding 1.5 to both sides. The 30th percentile of repair times is 2.25 hours. 30% of repair times are 2.5 hours or less.<\/li>\n<li><img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/h8bp-3l6e627i#fixme#fixme#fixme\" alt=\"\" \/><br \/>\nUniform Distribution between 1.5 and 4 with an area of 0.25 shaded to the right representing the longest 25% of repair times. [latex]P(x>k)[\/latex] = 0.25<br \/>\n[latex]P(x>k)[\/latex] = (base)(height) = [latex](4 \u2013 k)(0.4)0.25 = (4\u2013k)(0.4)[\/latex]; Solve for <em>k<\/em>: 0.625 = 4 \u2212\u00a0<em>k<\/em>, obtained by dividing both sides by 0.4<\/li>\n<\/ol>\n<p>\u22123.375 = \u2212\u00a0<em>k<\/em>, obtained by subtracting four from both sides: <em>k<\/em> = 3.375<\/p>\n<p>The longest 25% of furnace repairs take at least 3.375 hours (3.375 hours or longer).<\/p>\n<p><strong>Note:<\/strong> Since 25% of repair times are 3.375 hours or longer, that means that 75% of repair times are 3.375 hours or less. 3.375 hours is the <strong>75th percentile<\/strong> of furnace repair times.<\/p>\n<ul>\n<li>[latex]{\\mu}={\\frac{a+b}{2}}\\text{ and }{\\sigma}=\\sqrt{\\frac{(b-a)^2}{12}}[\/latex]<br \/>\n[latex]{\\mu}=\\frac{1.5+4}{2}=2.75\\text{ hours and }{\\sigma}=\\sqrt{\\frac{(4-1.5)^2}{12}}= 0.7217 \\text{ hours}[\/latex]<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Try It<\/h3>\n<p>The amount of time a service technician needs to change the oil in a car is uniformly distributed between 11 and 21 minutes. Let\u00a0<em>X<\/em> = the time needed to change the oil on a car.<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Write the random variable <em>X<\/em> in words. <em>X<\/em> = __________________.<\/li>\n<li>Write the distribution.<\/li>\n<li>Graph the distribution.<\/li>\n<li>Find [latex]P(x>19)[\/latex].<\/li>\n<li>Find the 50th percentile.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q962497\">Show Answer<\/span><\/p>\n<div id=\"q962497\" class=\"hidden-answer\" style=\"display: none\">\n<ol style=\"list-style-type: lower-alpha;\">\n<li>Let <em>X<\/em> = the time needed to change the oil in a car.<\/li>\n<li><em>X<\/em> ~ <em>U<\/em> (11, 21).<\/li>\n<li><img decoding=\"async\" src=\"https:\/\/textimgs.s3.amazonaws.com\/DE\/stats\/ld91-516e627i#fixme#fixme#fixme\" alt=\"This graph shows a uniform distribution. The horizontal axis ranges from 405 to 525. The distribution is modeled by a rectangle extending from x = 447 to x = 521.\" \/><\/li>\n<li>[latex]P(x>19) = 0.2[\/latex]<\/li>\n<li>the 50th percentile is 16 minutes.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-252\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>OpenStax, Statistics, The Uniform Distribution. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/openstax.org\/books\/statistics\/pages\/5-2-the-uniform-distribution\">https:\/\/openstax.org\/books\/statistics\/pages\/5-2-the-uniform-distribution<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/statistics\/pages\/1-introduction<\/li><li>Introductory Statistics. <strong>Authored by<\/strong>: Barbara Illowsky, Susan Dean. <strong>Provided by<\/strong>: Open Stax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/openstax.org\/books\/introductory-statistics\/pages\/1-introduction\">https:\/\/openstax.org\/books\/introductory-statistics\/pages\/1-introduction<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/introductory-statistics\/pages\/1-introduction<\/li><li>College Algebra. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/openstax.org\/books\/college-algebra\/pages\/1-introduction-to-prerequisites\">https:\/\/openstax.org\/books\/college-algebra\/pages\/1-introduction-to-prerequisites<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/college-algebra\/pages\/1-introduction-to-prerequisites<\/li><li>Unit 2: Fractions and Mixed Numbers, from Developmental Math: An Open Program. <strong>Provided by<\/strong>: Unit 2: Fractions and Mixed Numbers, from Developmental Math: An Open Program. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/nrocnetwork.org\/resources\/downloads\/nroc-math-open-textbook-units-1-12-pdf-and-word-formats\/\">http:\/\/nrocnetwork.org\/resources\/downloads\/nroc-math-open-textbook-units-1-12-pdf-and-word-formats\/<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":169134,"menu_order":11,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"OpenStax, Statistics, The Uniform Distribution\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"https:\/\/openstax.org\/books\/statistics\/pages\/5-2-the-uniform-distribution\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/statistics\/pages\/1-introduction\"},{\"type\":\"cc\",\"description\":\"Introductory Statistics\",\"author\":\"Barbara Illowsky, Susan Dean\",\"organization\":\"Open Stax\",\"url\":\"https:\/\/openstax.org\/books\/introductory-statistics\/pages\/1-introduction\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at 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