Seasonal variation in suicide occurrence in Finland.

The aim of this study was to investigate the age-, gender- and suicide method-related seasonality of suicide occurrence by using the largest database examined so far (n=21,279). The Chi-square test for multinomials was used as the overall measure of deviation. The monthly observed and expected numbers of suicides were calculated and classified by year, month, gender, age groups and suicide methods. To identify the statistically significant peak and trough months, the ratio of observed numbers of suicides to expected numbers with 95% confidence intervals was calculated. For males, there was a suicide peak from April to July, while for females the distribution was bimodal (with peaks in May and October). In elderly people there was a significant excess in the number of suicides in autumn, and the troughs were deeper in winter. For violent suicides there was a unimodal spring peak, but for non-violent suicides the distribution was bimodal. The results indicate that suicides among elderly subjects, as well as non-violent suicides, occur significantly more often during autumn than would be expected.