Maximizing control of the sample, setting, and procedures will enhance internal validity; however, increasing internal validity will limit external validity or the ability to generalize study results to other populations (Pedhazur & Schmelkin, 1991). For example, results of a study with a highly homogenous sample may not be generalizable to a sample with different characteristics. Thus, implications of design decisions are considered when planning a study. Is it more important to have a highly controlled study with high internal validity? Or will such a highly controlled study produce results that will not be relevant to clinical practice? An investigator needs to determine if internal validity is of the highest priority or if external validity should also be considered. More on instrument validity is found in the chapter Observational Studies by Richard Berk, PhD.
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