Measurement of individual differences: lessons from memory assessment in research and clinical practice.

An examination of test manuals and published research indicates that widely used memory tests (e.g., Verbal Paired Associates and Word List tests of the Wechsler Memory Scale, Rey Auditory Verbal Learning Test, and California Verbal Learning Test) are afflicted by severe ceiling effects. In the present study, the true extent of memory ability in healthy young adults was tested by giving 208 college undergraduates verbal paired-associate and verbal learning tests of various lengths; the findings demonstrate that healthy adults can remember much more than is suggested by the normative data for the memory tests just mentioned. The findings highlight the adverse effects of low ceilings in memory assessment and underscore the severe consequences of ceiling effects on score distributions, means, standard deviations, and all variability-dependent indices, such as reliability, validity, and correlations with other tests. The article discusses the optimal test lengths for verbal paired-associate and verbal list-learning tests, shows how to identify ceiling-afflicted data in published research, and explains how proper attention to this phenomenon can improve future research and clinical practice.