One technique for examining the relationship between variables is to observe
the two variables as they exist naturally for a set of individuals. That is, simply
measure the two variables for each individual. For example, researchers
have found a relationship between GPA and sleep habits, specifi cally, wake-up
time, for college students (Trockel, Barnes, & Egget, 2000). Figure 6.2 shows
an example of the kind of data found in the study.