Читать книгу Methods in Psychological Research - Annabel Ness Evans - Страница 96

As we just discussed, in partial correlation, we remove the relationship of one variable from the other variables and then calculate the correlation. But what if we want to remove the influence of a variable from only one of the other variables? This is called a semipartial correlation. For example, at our school, we accept senior students in our applied psychology program based on their grades in the first and second years. We have found a strong positive correlation between previous grades and performance in our program. Suppose we could also administer an entrance exam to use as another predictor, but the exam was expensive. We can use semipartial correlation to determine how much the entrance test will increase our predictive power over and above using previous grades.

How do we do this? Well, we correlate entrance test scores and performance in the program after first removing the influence of previous grades on program performance. This correlation value then will tell us what relationship remains between entrance test scores and program performance when the correlation with previous grades has been partialled out of program performance but not out of entrance test scores. In our example, we could decide, based on this correlation, whether an expensive entrance test helped our predictive ability enough for us to go ahead and use it.