Abstract
Seasonal variations of events are apparently playing an important part in various psychiatric conditions. To study the seasonal variation of a condition appears to be one useful approach to clarify the aetiology of a mental disorder and phenomena to which mental disorders are associated. In the present study the seasonal variations of suicides during the period of 1980-95 (n=21279) and homicides during the years 1957-95 (n=4553) in Finland were analysed. In addition, the use of statistical techniques for seasonality and some important characteristics of study samples were evaluated from 44 original suicide seasonality studies published between 1970-97. Special attention was paid to statistical methods for seasonality and these were reviewed in the summary part of this dissertation.
A statistically significant spring peak of suicides was found in both genders, in all age groups (aged 39 years or below, 40-64 years, and 65 years or more) and in violent (hanging, drowning, shooting, wrist-cutting, jumping from a height) and non-violent suicides (poisoning, gas, other methods). A secondary autumn peak of suicides was present in females and also associated with non-violent methods. The rate of violent suicides had increased significantly during 1980-90 and decreased thereafter, while the non-violent suicides had kept steadily increasing over the whole 16-year study period. The seasonal variation of violent suicides had remained stable and statistically significant over the whole study period, but the seasonality in non-violent suicides has diminished over time.
The seasonal pattern of homicides showed a statistically significant peak in summer and a trough in winter. The observed rate of homicides was about 6% higher in summer and 6% lower in winter than expected under the null hypothesis of a uniform distribution. Both the crude numbers of homicide and the rate of homicides per 100 000 population increased significantly over the 39-year study period. The increasing rate of homicides in Finland was accompanied by decreasing homicide seasonality. The seasonal trends in homicides correlated significantly (positive correlation) with the seasonal trends in the violent suicides over the period of 1980-95.
The use of particular statistical techniques was specified in the majority of the 44 reviewed suicide seasonality articles. This was considered as satisfactory, although in subgroup analyses and in comparisons of the seasonal pattern of suicides with phenomena other than suicides, researchers tended to interpret their study findings without a statistical significance test. In those 37 articles, which had actually examined the seasonal pattern of suicides with a statistical test, statistical methods varied from simple standard tests like the chi-square test (14 articles, 38%) to sophisticated time series analyses such as a spectral analysis (4 articles, 11%). The calendar effect (i.e. effect due to the unequal lengths of months and leap years) was reported to have been taken into account in only 10 out of 44 (22%) reviewed studies. The lack of reporting the size of a sample (12 articles, 27%) or monthly values of suicides (17 articles, 54%) was found to be a major deficit in the reviewed studies. On the basis of these findings it is recommended to carry out further surveys, which evaluate statistical content and use of statistical methods in published medical articles. These kinds of surveys remind researchers to consider more thoroughly methodological and statistical issues in their investigations.
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| Acknowledgements |