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APPENDIX B
Appendix B: Developing the concept of absolute risk
In this report, the concept of relative risk has been used, with little discussion of
what the absolute risk of murder (or SSA) is for individuals with particular criminal
histories. This is a deliberate strategy, as the absolute risk of murder is always
conditional on the population under consideration, and the future time horizon. For
example, estimating the absolute risk of those with a conviction for ‘other wounding
etc.’ of subsequent murder until age 40 for those born in 1953 would be different to
estimating the future lifetime absolute risk of the current criminal population. In
addition, large samples are needed to calculate absolute risk with any degree of
accuracy. However some progress can be made.
This problem has been approached by looking at the OI birth cohorts. The 1953
birth cohort contains the criminal histories of everyone born in one of four selected
weeks in 1953 and convicted of a standard list offence; similar cohorts exist for
1958, 1963, 1968, 1973 and 1978. This gives us a prospective data set, but one
subtly different to the dataset used in the report. First, many of the offenders in the
cohort samples are minor offenders (committing a single offence in their teens and
then desisting); this is somewhat different to the active criminals chosen for the
case-control study. Secondly, the later cohorts are young cohorts – the 1978 cohort
in particular is only followed up until 1999, the offenders in this cohort would be 22
or younger. For this reason, two later cohorts have been excluded.
Three sample offences have been considered here: other wounding etc., robbery and
kidnapping. Two methods of estimation of absolute risk were used: a direct method
based on the cohort data alone; and an indirect method using the estimated relative
risk from the case-control study.
Other wounding etc. (offence code 8)
The table below gives the number of murders and non-murders for those with and
without prior ‘other wounding’, and calculates the odds-ratio and the relative risk.
By combining all four cohorts together, it is possible to get a global estimate of
absolute risk over a medium term time horizon. Given a previous conviction, the
odds of murder given a prior ‘other wounding’ is estimated at one chance of murder
to 688 against, and the absolute risk of murder given prior ‘other wounding’ was
therefore estimated at 1 in 689. Similarly, the absolute risk of murder given no prior
‘other wounding’ was estimated at 1 in 1068. The estimate of the relative risk of
murder for prior ‘other wounding’ was the ratio of these two absolute risks:
1068/689, or 1.551. The relative risk estimated by the case-control method was
1.483, which represents a satisfying agreement.
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