title: Robustness of the t test applied to data distorted from normality by floor effects.
abstract: In calculus, plaque, and gingivitis trials, measures are taken on subjects both prior to the use of an active treatment and after its use. When the trial is short-term, or when a cleaning of the mouth takes place after the baseline measurement, distributions of such measures (e.g., the Volpe-Manhold score or the Löe and Silness scale) are approximately normally distributed above zero but also can have a proportion of subjects who attain scores of zero. When the effects of an active treatment are compared with those of a control, the two-independent-sample t test can be applied to outcome scores or to differences between the baseline and outcome scores. Robustness of these t tests, in the presence of distributions "distorted" from normality as described, was investigated by computer simulation. In general, both t tests produced actual significance levels which were close to nominal significance levels, even in the presence of small samples and distributions in which as many as 50% of the subjects attained scores of zero.
authors.affiliation: Boston University, Mathematics Department, Massachusetts 02215.
authors.forename: L M
authors.initials: LM
authors.lastname: Sullivan
authors.forename: R B
authors.initials: RB
authors.lastname: D'Agostino
mesh_terms: D002986:Clinical Trials as Topic; D003198:Computer Simulation; D006801:Humans; D015233:Models, Statistical; D011336:Probability; D012016:Reference Values; D013223:Statistics as Topic; D016896:Treatment Outcome