For any data set you are going to graph, you have to decide which of the two variables you are going to put on the x-axis and which one you are going to put on the y-axis. In graphing jargon, the independent variable is plotted on the x-axis and the dependent variable is plotted on the y-axis.
In any data set, the independent or X-variable is the one that was chosen or manipulated by the experimenter. For example, time is always an independent variable (and goes on the x-axis) because the experimenter is choosing which time points to take measurements at—1 second intervals, 5 minute intervals, etc. In drug trials, the drugs used are always the independent variable (and go on the x-axis) because the experimenter chose which drugs to test.
In any data set, the dependent or Y-variable is the one whose value depends on or is affected by the value of the independent variable. It is the one measured or reported by the researchers. For example, if you are following the number of cancer cases per year, you plot the number of cancer cases on the y-axis because that number is different for each year, and the value you plot depends on which year you choose (you are choosing the year, so year goes on the x-axis; you are reporting the number of cancer cases, which is dependent on which year you chose, so that goes on the y-axis.) The researchers are doing the research to find out the Y-variables. They chose the X-variables before the study started, but they don’t know the Y-variable data until they do their research.
Lab 2 Exercises 2.3
For each of the following data sets, which is the independent variable (X variable) and which is the dependent variable (Y variable)?
- You are testing the toxicity of a new drug. You administer different doses to groups of mice and determine the percentage of the group that died as a result. The two variables are:
- Percentage of dead mice ___________________________ x-axis or y-axis?
- Drug dosage ___________________________ x-axis or y-axis?
- You are following the crime rate in Columbus, MS, over the past year. You are counting the number of crimes in each month. The two variables are:
- Month ___________________________________ x-axis or y-axis?
- Number of crimes ___________________________________ x-axis or y-axis?
- You take all the crimes in Columbus, MS, in the past year and classify them according to type (murder, robbery, assault, etc.) The two variables are:
- Type of crime: _____________________________ x-axis or y-axis?
- Number of that type of crime _____________________________ x-axis or y-axis?
- You are comparing three different cold remedies. You have volunteers with colds take one of the remedies and you measure how much longer the cold lasts after taking the medicine.
- How much longer the cold lasts ____________________________ x-axis or y-axis?
- Which cold remedy was used ____________________________ x-axis or y-axis?