Striving to deconfound the fundamentals of epidemiologic study design.

The fundamentals of epidemiologic study design have remained a matter of confusion. Most authors still see the main design options to consist of the "cohort" study and the "case-control" study, augmented by the "cross-sectional" study. Others regard these as options only with respect to the perceived "directionality" dimension of design decisions. Few have come to appreciate that, realistically, there are no options as to directionality in the usual sense of "following forward" vs "investigating backward", or in the related sense of "inferential reasoning" being "from cause to effect" vs "from effect to cause". Related to this, few appreciate that the perceived duality of options constituted by "sampling by exposure" and "sampling by outcome" is, similarly, but an illusion. Old illusions like these confound the discernment of even those who, today, strive to deconfound the fundamentals of epidemiologic study design.