Ambidirectional studies--an extension of longitudinal studies in psychiatry.

Both longitudinal and retrospective studies have inherent weaknesses. With psychiatric mortality studies as an example of longitudinal research, this article presents a study design, the ambidirectional study, that combines some of the advantages and minimizes some of the weaknesses inherent in longitudinal and retrospective studies respectively. The method involves 2 phases: first, definition of a base population some of whom proceed to encounter an event of interest (e.g. death, suicide, illness), secondly, a retrospective comparison of those emerging from the same base population with or without the event of interest. The advantages of this design are feasibility and freedom from confounding coherent effects, since cases and controls enter the study at the same point. The difficulties are those of retrospective data collection. Definition of controls, case matching and statistical methodology are discussed.