A second way is to ask, with a given reliability coefficient, how well can you predict a second score by knowing the first? If one has a first set of scores for a group of students, a reliability coefficient of .90 indicates that these first scores allow you to predict about 80 percent of the variability in the second scores. With a reliability coefficient of .70, one can predict only about half of the variability in a second set of scores.
Scatterplot with furst score as x abd second score as y. R squared is measured.
