Learning Outcomes
- Interpret percentiles, medians and quartiles in context
Interpreting Percentiles, Quartiles, and Median
A percentile indicates the relative standing of a data value when data are sorted into numerical order from smallest to largest. Data value percentages are less than or equal to the [latex]p[/latex]th percentile. For example, [latex]15[/latex]% of data values are less than or equal to the [latex]15[/latex]th percentile.
- Low percentiles always correspond to lower data values
- High percentiles always correspond to higher data values
A percentile may or may not correspond to a value judgment about whether it’s “good” or “bad.” The interpretation of whether a certain percentile is “good” or “bad” depends on the context of the situation to which the data applies. In some situations, a low percentile would be considered good; in other contexts a high percentile might be considered good. In many situations, there is no value judgment that applies.
Understanding how to interpret percentiles properly is important not only when describing data, but also when calculating probabilities in later chapters of this text.
Guidelines
When writing the interpretation of a percentile in the context of the given data, the sentence should contain the following information.
- information about the context of the situation being considered
- the data value (value of the variable) that represents the percentile
- the percent of individuals or items with data values below the percentile
- the percent of individuals or items with data values above the percentile
Example
On a timed math test, the first quartile for time it took to finish the exam was [latex]35[/latex] minutes. Interpret the first quartile in the context of this situation.
Try It
For the [latex]100[/latex]-meter dash, the third quartile for times for finishing the race was [latex]11.5[/latex] seconds. Interpret the third quartile in the context of the situation.
Example
On a [latex]20[/latex] question math test, the [latex]70[/latex]th percentile for number of correct answers was [latex]16[/latex]. Interpret the [latex]70[/latex]th percentile in the context of this situation.
Try It
On a [latex]60[/latex] point written assignment, the [latex]80[/latex]th percentile for the number of points earned was [latex]49[/latex]. Interpret the [latex]80[/latex]th percentile in the context of this situation.
Example
At a community college, it was found that the [latex]30[/latex]th percentile of credit units that students are enrolled for is seven units. Interpret the [latex]30[/latex]th percentile in the context of this situation.
Try It
During a season, the [latex]40[/latex]th percentile for points scored per player in a game is eight. Interpret the [latex]40[/latex]th percentile in the context of this situation.
Example
Sharpe Middle School is applying for a grant that will be used to add fitness equipment to the gym. The principal surveyed [latex]15[/latex] anonymous students to determine how many minutes a day the students spend exercising. The results from the [latex]15[/latex] anonymous students are shown.
[latex]0[/latex] minutes; [latex]40[/latex] minutes; [latex]60[/latex] minutes; [latex]30[/latex] minutes; [latex]60[/latex] minutes
[latex]10[/latex] minutes; [latex]45[/latex] minutes; [latex]30[/latex] minutes; [latex]300[/latex] minutes; [latex]90[/latex] minutes;
[latex]30[/latex] minutes; [latex]120[/latex] minutes; [latex]60[/latex] minutes; [latex]0[/latex] minutes; [latex]20[/latex] minutes
Determine the following five values.
- Min = [latex]0[/latex]
- [latex]Q_1[/latex] = [latex]20[/latex]
- Med = [latex]40[/latex]
- [latex]Q_3[/latex] = [latex]60[/latex]
- Max = [latex]300[/latex]
If you were the principal, would you be justified in purchasing new fitness equipment?
