To Pretest or Not to Pretest

Robert Pasnak*

Received: April 23, 2018;   Published: June 08, 2018

DOI: 10.26717/BJSTR.2018.05.001185

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Abstract

The place of taking measures on a dependent variable before treatment, hereafter labeled pretesting, in any type of research is described. It is particularly issues when behavior of the patients or subjects is a key interest. Pretests decrease the internal validity of experiments (i. e., render them incapable of completely proving what they are intended to prove). The loss of internal validity may be great in some cases, and little or no loss in others, but just how much has been lost cannot be proven. Pretests are sometimes credited with demonstrating that randomization has produced reasonably equivalent groups, but that is indexed by p, with equal accuracy whether or not pretests are employed. There are usually many sources of differences between subjects which could result in different outcomes on the dependent variable(s). The only valid index of whether an initial inequality of groups has produced a spurious result is p. This value specifies the probability that differences in the outcome for various levels of the independent variable(s) were produced by a failure of randomization to create equality in whatever pretests might measure and any other source of differences. Pretests can increase the power of experiments to detect small differences in outcomes, but only if pretest and posttest scores are strongly correlated. Conditions under which accepting the confound introduced by pretesting is most likely to be profitable are identified..

Keywords: Pretests; Internal Validity; External Validity; Power

Abstract| Introduction| Pretest-treatment Interactions| When to Pretest and When Not| References|