Threats to internal validity in worksite health promotion program research: common problems and possible solutions.

Reviews of the research on worksite health promotion programs reveal that most studies are limited in their ability to draw clear inferences about program effects because the studies employed flawed research designs and/or analyses. Conclusions are often drawn about program effectiveness with little consideration given to alternative explanations for the findings. In an effort to promote improved research, this article uses the Cook and Campbell delineation of threats to valid causal inference to illustrate how the threats can operate in worksite health promotion program research as well as how they can be examined and controlled. Researchers, even those attempting to conduct true experiments, must consider the existence of all plausible threats to validity and control or rule them out before valid causal inferences can be drawn. The theoretical and design issues involved in worksite health promotion program research are presented, followed by a discussion of threats to internal validity.