Jupiter pilots his newly created perfectionism scale on a high school psychology class. He returns one month later to administer the same test to the same students, and then he correlates the two sets of results. What is Jupiter probably doing?

(E) measuring the test’s reliability

In correlating the two sets of results, Jupiter is measuring the test’s reliability. This type of reliability is known as test-retest reliability. Since perfectionism should not change over the course of the month, a low correlation would indicate that the test was not reliable. Outliers are extreme scores, and Jupiter does not appear to be looking for them. To standardize the test, Jupiter would have to give it to a standardization sample and then look to assemble a group of questions the yielded a normal distribution of scores. Nothing in the question suggests Jupiter believes the mean level of perfectionism should have changed, and the correlation will not necessarily show whether it has. Finally, even though Jupiter’s work will show whether or not the test is reliable, it will not show whether or not it is valid. Validity is a measure of accuracy—in this case, whether the test actually measures perfectionism.