The right answer for the wrong question: Consequences of type III error for public health research

Objectives. This study examined the impact of assessing the causes of inter individual variation within a population when the research question of inte rest is about causes of differences between populations or time periods. Th is discrepancy between the research focus and the research question is refe rred to as a type III error, one that provides the right answer for the wro ng question. Methods. Homelessness, obesity, and infant mortality were used to illustrat e different consequences of type III errors. These different consequences d epend on the relationships between the causes of within- and between group variation. Conclusions. The causes of inter-individual variation and the causes of var iation between populations and time periods may be distinct. The problem of examining invariant causes deserves attention.