Abstract n Reply.— Psychiatric classification remains a major problem because it must still be inferred from semiquantitative ratings of clinical phenomena. Much the same problem confronted biologists attempting to distinguish discrete species on the basis of morphology in the late 19th century, resulting in many inconsistent classifications. Then, in 1894, Karl Pearson developed statistical methods to identify subpopulations from a heterogeneous mixture.1 The existence of two subgroups can be inferred under some conditions when quantitative data have a bimodal distribution, ie, two peaks separated by an intermediate point of rarity. Later, many species were successfully distinguished in this way.2 However, bimodality may also arise from a variety of measurement and selection artifacts, such as the arbitrary exclusion of intermediate cases or the pooling of nonrandom subsamples of extreme cases.3 Accordingly, the inference of multiple discrete disorders from quantitative data requires careful attention to experimental design and statistical analysis. References 1. Pearson K: Contribution to the mathematical theory of evolution . Phil Trans R Soc Lond 1894;185:71-110.Crossref 2. Kevles DJ: In the Name of Eugenics: Genetics and the Uses of Human Heredity . New York, Alfred A Knopf Inc, 1985, pp 27-29. 3. Murphy EA: One cause? Many causes? The argument from the bimodal distribution . J Chron Dis 1964;17:301-324.Crossref 4. Cloninger CR, Sigvardsson S, von Knorring A-L, Bohman M: An adoption study of somatoform disorders: II. Identification of two discrete somatoform disorders . Arch Gen Psychiatry 1984;41:863-871.Crossref 5. Cloninger CR, Martin RL, Guze SB, Clayton PJ: Diagnosis and prognosis in schizophrenia . Arch Gen Psychiatry 1985;42:15-25.Crossref 6. Vogel F, Motulsky AG: Human Genetics: Problems and Approaches . Berlin, Springer-Verlag, 1979, pp 481-483.
Archives of General Psychiatry – American Medical Association
Published: Jul 1, 1986