Justifying a "fast acting" claim for antidepressant drugs.

Justification of a "fast acting" claim for antidepressant drugs is, first and foremost, a definition problem. The one controversial statistical issue that has been raised is whether time should enter the equation as an independent or dependent variable. Although alternative models provide essentially equivalent tests of significance and estimates of response latencies, a regression model--in which assessment times are recognized to be fixed independent variables and repeated quantitative measurements of clinical response are the dependent variable--is more congruent with the determinate experimental conditions and sources of error variation in antidepressant drug trials. The regression model gains power for tests of significance and precision for estimates of response times by utilizing fully the information contained in quantitative repeated measurements rather than considering only time to an artificially discrete endpoint.