Fractional factorial designs for legal psychology

Researchers considering novel or exploratory psycholegal research are often able to easily generate a sizable list of independent variables (IVs) that might influence a measure of interest. Where the research question is novel and the literature is not developed, however, choosing from among a long list of potential variables those worthy of empirical investigation often presents a formidable task. Many researchers may feel compelled by legal psychology's heavy reliance on full-factorial designs to narrow the IVs under investigation to two or three in order to avoid an expensive and unwieldy design involving numerous high-order interactions. This article suggests that fractional factorial designs provide a reasonable alternative to full-factorial designs in such circumstances because they allow the psycholegal researcher to examine the main effects of a large number of factors while disregarding high-order interactions. An introduction to the logic of fractional factorial designs is provided and several examples from the social sciences are presented.