Identifying probable suicide clusters in wales using national mortality data

Abstract

Background: Up to 2% of suicides in young people may occur in clusters i.e., close together in time and space. In early 2008 unprecedented attention was given by national and international news media to a suspected suicide cluster among young people living in Bridgend, Wales. This paper investigates the strength of statistical evidence for this apparent cluster, its size, and temporal and geographical limits.

Methods and findings: The analysis is based on official mortality statistics for Wales for 2000-2009 provided by the UK's Office for National Statistics (ONS). Temporo-spatial analysis was performed using Space Time Permutation Scan Statistics with SaTScan v9.1 for suicide deaths aged 15 and over, with a sub-group analysis focussing on cases aged 15-34 years. These analyses were conducted for deaths coded by ONS as: (i) suicide or of undetermined intent (probable suicides) and (ii) for a combination of suicide, undetermined, and accidental poisoning and hanging (possible suicides). The temporo-spatial analysis did not identify any clusters of suicide or undetermined intent deaths (probable suicides). However, analysis of all deaths by suicide, undetermined intent, accidental poisoning and accidental hanging (possible suicides) identified a temporo-spatial cluster (p = 0.029) involving 10 deaths amongst 15-34 year olds centred on the County Borough of Bridgend for the period 27(th) December 2007 to 19(th) February 2008. Less than 1% of possible suicides in younger people in Wales in the ten year period were identified as being cluster-related.

Conclusions: There was a possible suicide cluster in young people in Bridgend between December 2007 and February 2008. This cluster was smaller, shorter in duration, and predominantly later than the phenomenon that was reported in national and international print media. Further investigation of factors leading to the onset and termination of this series of deaths, in particular the role of the media, is required.

Figure 1. Ten year rates of probable suicides (suicides and undetermined deaths) for Welsh Counties 2000–2009.

The geographical variations in rates of probable suicide across Welsh counties 2000 to 2009.

Figure 2. Mortality statistics for Wales by cause of death group (rate per 100,000 population).

Apart from a small rise in 2003, deaths coded as suicide and of undetermined intent decreased between 2000 and 2009 in Wales as a whole (Figure 2a). Deaths coded as accidental poisoning and hanging decreased slightly from 2000 to 2005, before starting to increase in 2006 and rising by 61% in 2008. For possible suicides there were two peaks in the period: the first in 2003, which was due to a rise in deaths coded as suicide and of undetermined intent; and the second in 2008, which was due to a rise in deaths coded as accidental poisoning or accidental hanging. For ages 15 to 34 years there is a similar pattern but with greater variation (Figure 2b).

Figure 3. Mortality statistics for the County Borough of Bridgend by cause of death group (rate per 100,000 population).

The rate of death by cause across time for the County Borough of Bridgend (Figure 3a) was considerably more varied than the rate of death by cause across Wales, as might be expected considering the relatively small population of Bridgend. In the all-age rates (figure 3a) there was an indication of a peak in accidental deaths by hanging and poisoning in 2008 and some evidence that 15–34 year olds contributed disproportionately to this excess (figure 3b). In 15–34 year olds, possible suicides peaked in 2008 (figure 3b) with a second smaller peak in 2003.

Figure 4. Temporo-spatial clusters of possible suicides (suicide, undetermined, and accidental poisoning and hanging) for people 15–34 years.

The temporo-spatial analysis identified several possible clusters across Wales for age group 15 to 34 years. Three of the clusters were centred on the county borough of Bridgend, but only cluster number one was statistically significant (p = 0.029).

Figure 5. Most likely temporo-spatial clusters of possible suicide in Bridgend and surrounding boroughs 2000–2009.

The three most likely clusters for the age group 15 to 34 years centred on the county borough of Bridgend. The clusters all extended into neighbouring boroughs.