The use of effect sizes to characterize the nature of cognitive change in psychopharmacological studies: an example with scopolamine

Drug induced cognitive change is generally investigated using small sample sizes. In terms of null hypothesis significance testing (NHST) this can render a meaningful change non-significant, as a result of insufficient power in the statistical model. NHST leads to 'all or none' thinking, where a non-significant result is interpreted as an absence of change. An effect size calculation indicates the magnitude of change which has occurred post-intervention, and therefore whether a significant result is meaningful. We used a scopolamine challenge to demonstrate the usefulness of effect sizes. The aim of the study was to determine how effect sizes could describe the cognitive changes that occur following administration of subcutaneous scopolamine (s.c. scopolamine). Twenty four healthy young males (M = 32.6, sd = 4.5 years) were administered placebo and 0.2 mg, 0.4 mg & 0.6 mg of s.c. scopolamine using a 4-way crossover design. Memory, learning, psychomotor function, attention and executive function were assessed. Scopolamine significantly impaired performance on all tasks in a dose and time related manner. These results demonstrate the functionality of change scores to draw comparisons between different times and doses. This methodology overcomes the limitations of comparisons between studies using different tasks, doses and time at which cognitive functions are measured. Copyright (c) 2008 John Wiley & Sons, Ltd.