In the previous example, we were interested in seeing whether studying is associated with on test
performance. To do this we drew a scatterplot , which is just a two-dimensional graph of ordered pairs.
We put one variable on the horizontal axis and the other on the vertical axis. In the example, the horizontal
axis is for “hours studied” and the vertical axis is for “score on test.” Each point on the graph represents
the ordered pair for one student. If we have an explanatory variable , it should be on the horizontal axis,
and the response variable should be on the vertical axis.
In the previous example, we observed a situation in which students with higher values on the vertical
axis tend to have higher values on the horizontal axis. We say that two variables are positively associated
if one of them increases as the other increases and negatively associated if one of them decreases as the
other increases.
Calculator Tip: In order to draw a scatterplot on your calculator, first enter the data in two lists, say
the horizontal-axis variable in L1 and the vertical-axis variable in L2 . Then go to STAT PLOT and
choose the scatterplot icon from Type . Enter L1 for Xlist and L2 for Ylist . Choose whichever
Mark pleases you. Be sure there are no equations active in the Y = list. Then do ZOOM ZoomStat
(Zoom-9) and the calculator will draw the scatterplot for you. The calculator seems to do a much
better job with scatterplots than it does with histograms but, if you wish, you can still go back and
adjust the WINDOW in any way you want.
The scatterplot of the data in the example, drawn on the calculator, looks like this (the window
used was [0, 6.5, 1, 40, 105, 5, 1]):