These differences might show up as “longitudinal changes” when the
results of the first and second tests are compared, even though theyhave nothing to do with the aging process.
On the other hand, some critics have pointed out that individuals
often like to give an impression of consistency . They may recall whatthey said the first time when asked to respond again, and they may tryto give the same answers. This would give the impression of stabilityeven in the face of real change.
Test makers are painfully aware of this problem and have come up
with a few partial solutions. One is to use a different but similar test thesecond time—a so-called parallel form . This is the solution adopted, for
example, by the Educational Testing Service when students take theScholastic Aptitude Test (SAT) on more than one occasion. Since eachtest is different in content, there is no direct possibility of rememberingthe questions and figuring out the answers between tests. But most stu-dents will score a little higher the second time they take the test, proba-bly because they feel more comfortable with the process. Parallel formtesting is not a perfect solution.
If we are really concerned with the growth and change of a particu-
lar individual, the problem of practice effects is unavoidable. But if weare only interested in finding out whether people change with age, analternative strategy has been advocated by a number of prominentdevelopmental methodologists (P . B. Baltes, Reese, & Nesselroade,1977; Schaie, 1977). Suppose we take a random sample of individualsborn in 1920 and randomly divide them into two groups. We knowfrom the laws of statistics that, if we have a large enough sample, thetwo halves are unlikely to be different on any variable we choose tomeasure; they are equally representative of the population of men andwomen born in 1920. Next, suppose we measure the personality of thefirst group in 1980 but do nothing to the second group. If we return in2000 to measure the first group, we will have the standard longitudinaldesign and we will never quite know if the differences we find (or fail tofind) are the result of aging or of practice effects. However, if in 2000 wemeasure the second group and compare it with the first group as mea -
sured in 1980, the difference cannot be the result of practice (because
the second group had not taken the test before) and may therefore be aconsequence of aging itself. (Incidentally , if we measure both groups in2000, we can compare changes for the groups with and without practiceand so estimate what the pure practice effects are. That is extremelyuseful information for other investigators who need to know whether72 PERSONALITY IN ADULTHOOD