Chapter 3 - Health
               
Table 3.4 Number of Suicides, Standardised Mortality Ratios and Age Specific Mortality Ratios
                     1984 - 2007, males only
To enable comparisons with the number of suicides in the UK population, Standardised Mortality Ratios (SMR), adjusted for age and year, have been calculated. An SMR is defined as the ratio of the number of deaths observed in the study population (UK Armed Forces) to the number of deaths expected if the study population had the same age group and year specific rates as the standard (UK general population). An SMR over (or under) 100 indicates a higher (or lower) number of observed deaths than expected (based on standard population rates). An SMR of 100 implies that there is no difference in rates when comparing the UK Regular Armed Forces population with the UK population.

95% confidence intervals (95% CI) were calculated based on the Normal approximation, except where the number of observed events was less than 30, when they were derived directly from the Poisson distribution. If independent samples of the same size are repeatedly drawn from a population, with a 95% confidence interval calculated from each sample, then 95% of these intervals should contain the population mean. The width of the confidence interval gives us some idea of how uncertain we should be about the unknown parameter. Smaller samples result in wider confidence intervals, whereas larger and more representative samples will give narrower confidence intervals (providing greater accuracy).

The mortality rate is scaled for the standard population to be 100 (here the standard population is the UK population). The same scaling is used when measuring the mortality rate for the population being considered. If the mortality rate for this population is the same as for the UK population the underlying (true) mortality rate would be 100. The calculation of the rate based on data for a sample of the population over a restricted time period gives an estimate of the true rate. The observed mortality rate can be used to test the null hypothesis that the true mortality rate for this sample = 100 (the same as for the UK population) against the alternative hypothesis that it is not equal to 100 (differs from the UK population), using a 5% significance level.

When calculating a 95% confidence interval for the mortality rate, the lower and upper end-points of the interval are calculated from the observed value, giving 95% confidence that the true value lies within this interval. There is an exact relationship between the test and confidence interval. If there are very few deaths in the time period used to estimate the mortality rates, the estimate for the mortality rate will not be accurate (its standard deviation will not be small).

For each Service, and for the UK regular Armed Forces as a whole, the overall SMR was statistically significantly lower than expected on the basis of rates in the UK general population. For the Naval Service the SMR was 47 (95% CI=39-56) compared to the UK standard of 100, for the Army the SMR was 70 (95% CI=64-77) and for the RAF the SMR was 46 (95% CI=39-54).

For each Service and for the UK regular Armed Forces as a whole, the age-specific mortality ratios for each age group were also lower than expected on the basis of rates among the UK general population, with the exception of young Army males aged under 20 years. These young Army males have experienced 50% more suicides over the period 1984-2007 than their UK general population counterparts, a finding which is statistically significant.

Age in years Total <20 20-24 25-29 30-34 35-39 40+
Total 694 81 214 147 104 87 61
      Standardised Mortality Ratio 58 124 71 52 49 50 41
            95% Confidence Interval (54-63) (100-155) (62-81) (44-61) (40-59) (40-61) (32-53)
 
              
Naval Service
117 7 29 23 22 24 12
      Standardised Mortality Ratio 47 63 48 39 46 57 38
            95% Confidence Interval (39-56) (25-129) (32-70) (25-59) (29-69) (37-85) (20-67)
 
              
Army
421 68 153 83 50 43 24
      Standardised Mortality Ratio 70 150 87 57 49 55 47
            95% Confidence Interval (64-77) (118-190) (75-102) (46-70) (37-65) (40-74) (30-70)
 
              
Royal Air Force
156 6 32 41 32 20 25
      Standardised Mortality Ratio 46 69 47 52 50 37 40
            95% Confidence Interval (39-54) (25-150) (34-67) (38-70) (35-70) (22-56) (26-59)
Source: DASA (Health Information)