Department of Statistics and Actuarial Science, The University of Hong Kong
Institute of Statistics, National Tsing Hua University, Hsinchu, Taiwan
Department of Statistics, Harvard University, MA, USA
Correspondence: Paul Yip, Department of Statistics and Actuarial Science, The University of Hong Kong, Pokfulam Road, Hong Kong
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ABSTRACT |
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Aims To re-examine the seasonal variation in suicides in England and Wales for the period 1982-1996.
Method A harmonic analysis was used to detect the seasonality of the suicide data.
Results The seasonal effect on suicide is greatly diminished in England and Wales. This is shown by the reduced amplitude and smaller proportion of variance accounted for by the season.
Conclusions The seasonal effect on suicide has either diminished or vanished.
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INTRODUCTION |
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METHOD |
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The seasonal variations in suicides were examined in three ways. First, the
number of suicides in England and Wales for each month of the period 1982-1996
was plotted separately for each gender. Second, a daily mean suicide incidence
and cumulative number of suicides were calculated for each month of the study
period. Third, a harmonic time series model was applied to different genders,
age groups and methods of suicide. The method of analysis was similar to that
employed by Barraclough & White
(1978), Micciolo et al
(1991), Ho et al
(1997) and Yip et al
(1998). In this model, the
variation in suicides between the months is described as the sum of the
sinusoidal curves. The seasonal variation consists of those components with
cycles that repeat themselves an exact number of times per year. Let
Ai be the number of suicides for month i; for a
period of 15 years (180 months),
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RESULTS |
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Table 2 shows the results of
harmonic analysis by gender. Only 15 and 17% of the total variations can be
explained by the seasonal component for males and females, respectively. The
effects of all seasonal harmonics are marginally statistically significant,
with P0.05. In order to examine the possible determinant
factors of season by different subgroups, the figures for age and methods of
suicide were also subjected to the same analysis. The results obtained on the
seasonal components (not shown here) are not statistically significant.
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DISCUSSION |
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Reduction in seasonal fluctuation observed in other countries
Similar findings using the same harmonic analysis were obtained in other
countries: for example, Micciolo et al
(1989) suggested that 48-65% of
the variance in Italy's suicide data in the 1970s was explained by seasonal
components; in the 1980s and 1990s this figure was only 25-32% in Hong Kong
and Taiwan (Ho et al,
1997), and 3-17% in Australia and New Zealand
(Yip et al,
1998).
Significant reduction in suicides among the widowed and divorced
The reduction of the seasonal component can be explained by the significant
reduction in suicides among the widowed and divorced in England and Wales for
the past two decades. In fact, the decrease in suicide rates in England and
Wales from the widowed and divorced groups accounts for most of the overall
fall in the suicide rate for men and women. For example, the suicide rates for
males and females aged 60 years or over have fallen from 18.2 to 11.1 and from
10.9 to 4.0, respectively, for the period 1982-1996. The precise reasons for
the decrease among the widowed and divorced, especially among the elderly, are
yet to be determined. It has been shown that in Finland the autumn peak is
related to marital status. There was a high frequency of suicides among the
divorced (and possibly widowed) in September. Furthermore, divorced males, who
are known to be located in a lower than average position in the social scale,
might be claimed to be liable to suicide in autumn, owing to the increased
unemployment rate at that time of the year
(Nayha, 1983; Yip, 1997). Reduction of
suicide for the two groups would have an impact on the seasonal variations in
suicides.
Change of lifestyle
Durkheim (1897) believed
that seasonal variation in suicidal behaviour was determined by the intensity
of communal life and activities. Technological development in
telecommunications and the fact that people tend to be connected
more often than before (for example, by mobile phones, e-mail and the
internet) and also the different activities and social contacts nowadays, all
may play a part in determining the seasonal effect on suicide.
The exact impact of these changes on the seasonal variation in suicides is far from clear, but it is not surprising to see a reduction in the seasonal influence on the distribution of suicides.
In conclusion, the present study found that the seasonal effect on suicides in England and Wales is less obvious than previous studies suggested. The present finding challenges the existence of seasonal effects and predicts that seasonal variation in suicides will disappear in the new millennium.
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Clinical Implications and Limitations |
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LIMITATIONS
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ACKNOWLEDGMENTS |
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REFERENCES |
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Received for publication September 6, 1999. Revision received May 17, 2000. Accepted for publication May 18, 2000.