To Pretest or Not to Pretest
Journal Title: Biomedical Journal of Scientific & Technical Research (BJSTR) - Year 2018, Vol 5, Issue 2
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. There are three great goals for all research: internal validity, external validity, and power. An unfortunate misunderstanding that has become increasingly prevalent is the conception that pretesting increases internal validity. In fact, pretesting has the opposite effect; it reduces internal validity. What pretesting does increase is power if pretest and posttest scores are strongly correlated. Small effects may be detected if pretests are administered, although there are issues in statistical analysis after pretesting. This may reflect displacement of conventional methodology courses by courses in advanced correlational methods, which have great value in their own right and have engaged a generation of young scholars. This brief treatise is an effort to clarify the purpose and interpretation of pretests. To make it easier to follow, the discussion will be in terms of one independent and one dependent variable, although good research often involves more than one of each, andthe measurements taken for the dependent variable will defined as participants scores, although the same considerations hold for other measurements. Internal validity expresses the extent to which we can be certain that, within the conditions of the experiment, the effects attributed to the independent variable are indeed due to that variable and not some other. Any variable whose effects might be mistaken for those of the independent variable is termed a “confound” or confounding variable. Confounds are sometimes inherent in the design of an experiment – as when pretests are employed – or are sometimes produced when an experiment is not conducted properly. When unconfounded designs are used and experiments are conducted properly, there will be no confounds whose effects could be mistakenly attributed to the independent variable. A common cause of mistakes is the failure of random assignment of participants to levels of the independent variable to accomplish what it is supposed to accomplish - equality in all causal variables, known and unknown. The extent to which to which a failure of randomization could have produced the differences observed on the dependent variable is expressed by p. Pretests neither increase nor decrease the validity or accuracy of p as a measure of the failure of randomization to produce initial equality in the characteristics of participants in educational research. In essence a p of 02 means exactly the same thing whether or not there were pretests.
Authors and Affiliations
Robert Pasnak
Keywords
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- EP ID EP591664
- DOI 10.26717/BJSTR.2018.05.001185
- Views 165
- Downloads 0
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