The conceptual foundations for some of the most widely used research designs are presented, and the various design models are analyzed. Research designs should specify the stimulus and response, the time-order sequence, the sampling and allocation procedures, and the generalized method of analysis. Results of symbolization, seriation, and classification investigations cannot be generalized beyond the study units investigated. Cohort or case-control investigations prevent determination of causality. Cause can be logically proven if the design uses random assignment of study units to study groups and of study groups to stimulus conditions. Crossover designs offer no advantages over similarly constructed noncrossover designs, and introduce new sources of invalidity.