Objectives

To estimate the incapacitation effect and the impact on post-release recidivism of a measure combining prolonged incarceration and rehabilitation, the ISD measure for high frequency offenders (HFOs) was compared to the standard practice of short-term imprisonment.

Methods

We applied a quasi-experimental design with observational data to study the effects of ISD. The intervention group consisted of all HFOs released from ISD in the period 2004–2008. Two control groups were derived from the remaining population of HFOs who were released from a standard prison term. To form groups of controls, a combination of multiple imputation (MI) and propensity score matching (PSM) was used including a large number of covariates. In order to measure the incapacitation effect of ISD, the number of convictions and recorded offences in a criminal case of the controls were counted in the same period as their ISD counterfactuals were incarcerated. The impact on recidivism was measured by the prevalence and the frequency of reconvictions corrected for time at risk. Robustness of the results were checked by performing a combined PSM and difference-in-difference (DD) design.

Results

The estimate of the incapacitation effect was on average 5.7 criminal cases and 9.2 offences per ISD measure. On average 2.5 convictions and 4 recorded offences per year per HFO are prevented. The HFOs released from ISD showed 12 to 16 % lower recidivism rates than their control HFOs released from prison (Cohen’s h = 0.3–0.4). The recidivists of the ISD group also showed a lower reconviction frequency than the control group recidivists (Cohen’s d = 0.2).

Conclusions

The ISD measure seems to be effective in reducing recidivism and crime. The estimated incapacitation effect showed that a large portion of criminal cases and offences was prevented. DD analysis and sensitivity analyses confirmed the robustness of the PSM results. Due to the absence of actual treatment data, the effects found cannot be attributed separately to resocialization, imprisonment, or improvement of life circumstances.