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INTRODUCTION
● the number of offences for which an individual was convicted (an offender can
potentially have many offences at a single conviction date) and specific measures
of criminal convictions at an early age; and
● periods of custody.
Analysis
Taking the above summary measures of prior offending history as individual risk
factors, a matched case-control analysis was undertaken on each case-control group
using conditional logistic regression. In this context, the case-control study
examines the ratio of the odds of committing a serious offence with a risk factor
present in the criminal history to the odds of committing that offence without the
risk factor present 6,7 . For each potential risk factor (acting alone) and for each casecontrol
group combination, the odds-ratio of the risk factor on murder or SSA was
ascertained. For rare events (murder and SSA are both rare events), the odds-ratio
and the relative risk are nearly identical, and one can be taken as an approximation
for the other (Schlesselman 1982, p.33). The conditional logistic regression
produces estimates of odds-ratios, but as relative risks are easier to understand and to
interpret, they are reported as relative risks.
Following standard convention, the statistical tests used are based on a significance
level of 5 per cent (that is, any differences noted have a probability of only 5% or
less of occurring by chance). In presenting relative risks in the following chapters,
we do not report findings that fail to reach statistical significance.
The final issue relates to the assessment of absolute risk. In general, the issue of
absolute risk is harder to handle, as the risk depends on the population under study
(for instance young offenders or 30-year-old males) and the time period used.
Furthermore, much larger sample sizes would be needed to assess the absolute risk of
murder. Nevertheless, some illustrative calculations have been produced, shown as
Appendix B, based on a prospective dataset, the Home Office OI Cohort samples.
The samples
For murder cases, the basic sample consists of males, convicted of murder for the first
time between 1995 and 1997 in England and Wales. All those with a conviction for
murder (offence code 1) in these years were selected from the OI. This gave a total of
711 convicted murderers. Four males with previous murder convictions and 40 female
offenders were removed from the sample. The OI does not contain information on
7
6 The ‘odds’ of an event occurring
is calculated as simply the number
of chances for that event
happening divided by the number
of chances against it happening.
7 Both Breslow and Day (1980)
and Schlesselman (1982) show
that with a matched design,
simple methods of calculating
odds-ratios cannot be used, and
conditional logistic regression
must instead be applied. Analysis
was carried out using the
statistical software package
GLIM. Swan and Francis
(1992) showed how to use
GLIM to fit conditional logistic
regression models to matched
case-control data.