Effects of lorazepam on psychomotor performance: a comparison of independent-groups and repeated-measures designs

The purpose of this study was to compare the sensitivity to the effects of lorazepam (2.5 mg) of a design using independent groups (random allocation of subjects to either a placebo or a lorazepam treatment) with a repeated-measures design (subjects tested both before and after lorazepam treatment). With both designs, it was possible to demonstrate significant and equal effects of lorazepam in tests based upon speed of responding: Lorazepam significantly increased simple reaction time and significantly decreased performance in number cancellation and symbol copying tasks. The independent-groups design was more sensitive (i.e., showed effects at a higher level of significance) to the lorazepam-induced impairment in episodic memory, as assessed in a picture recognition task, and to the lorazepam-induced impairment in a word completion task. Comparisons between the two control condition scores indicated that there were unlikely to be significant group differences with random allocation of a relative homogeneous group of volunteers, such as medical students. While either design would be appropriate for homogeneous populations, for a heterogeneous clinical population where groups cannot be matched the repeated-measures design would be preferable.