Variability in psychometric test scores: the importance of the practice effect in patient study design.

1. Practice effect was determined for six types of psychometric tests given on eight occasions. 2. No plateau occurred. Stepwise mean decrease in Choice Reaction Time, Short Term Memory Errors, and Card Sorting Time, and stepwise mean increase in Mathematical, Visual Vigilance, and Digit-Symbol Substitution scores developed progressively. 3. Pre-trial training will usually fail to prevent practice effect from confounding effect of any intervention. 4. Design of drug intervention studies must take account of the continuing practice effect.