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Studies on month of birth of schizophrenic patients have consistently shown that more patients were born in the first four months of the year (January-April) and fewer in the second four months (May-August) than would be expected from the birth rate of the total population. The results reported by seven separate investigators are presented in Table 1. The fact that seven separate investigators have all found results in the same direction is statistically very conclusive. There is only one chance in 64 (P<0.02) that seven independent studies will show a difference between two groups in the same direction by chance alone, without the existence of a genuine difference. This estimate of probability is based on the very conservative assumption that each study has a 50-50 probability of showing a difference in either direction, just as a single coin-toss has a 50-50 chance of showing
Herbert Barry (Thu,) studied this question.