Practice effects in a longitudinal, multi-center Alzheimer’s disease prevention clinical trial
1 Sanders-Brown Center on Aging, University of Kentucky, Lexington, KY, USA
2 Departments of Neurology, University of Kentucky Lexington, Kentucky, USA
3 Departments of Biostatistics, University of Kentucky, Lexington, KY, USA
4 Departments of Statistics, University of Kentucky, Lexington, KY, USA
5 Departments of Behavioral Science, University of Kentucky, Lexington, KY, USA
6 Departments of Psychiatry, University of Kentucky, Lexington, KY, USA
7 Cancer Research and Biostatistics, Seattle, WA, USA
8 Sanders-Brown Center on Aging, University of Kentucky College of Medicine, Lexington, KY, USA
Trials 2012, 13:217 doi:10.1186/1745-6215-13-217Published: 20 November 2012
Practice effects are a known threat to reliability and validity in clinical trials. Few studies have investigated the potential influence of practice on repeated screening measures in longitudinal clinical trials with a focus on dementia prevention. The current study investigates whether practice effects exist on a screening measure commonly used in aging research, the Memory Impairment Screen (MIS).
The PREADViSE trial is a clinical intervention study evaluating the efficacy of vitamin E and selenium for Alzheimer’s disease prevention. Participants are screened annually for incident dementia with the MIS. Participants with baseline and three consecutive follow-ups who made less than a perfect score at one or more assessments were included in the current analyses (N=1,803). An additional subset of participants with four consecutive assessments but who received the same version of the MIS at baseline and first follow-up (N=301) was also assessed to determine the effects of alternate forms on mitigating practice. We hypothesized that despite efforts to mitigate practice effects with alternate versions, MIS scores would improve with repeated screening. Linear mixed models were used to estimate mean MIS scores over time.
Among men with four visits and alternating MIS versions, although there is little evidence of a significant practice effect at the first follow-up, mean scores clearly improve at the second and third follow-ups for all but the oldest participants. Unlike those who received alternate versions, men given the same version at first follow-up show significant practice effects.
While increases in the overall means were small, they represent a significant number of men whose scores improved with repeated testing. Such improvements could bias case ascertainment if not taken into account.