Research Question How did the use of diversion from prosecution and criminal sentencing change in Victoria, Australia in the ten years to 2016/17, with what estimated effects on repeat offending?
Data We tracked 1,163,113 criminal cases brought against both juveniles and adults by police in the state of Victoria, Australia, including 181,836 diversions, during the ten-year time period from the fiscal year of 2007/8 through 2016/17. Methods Taking the percentage of all cases diverted in the first year (25.6%), we calculated for each of the study years how many more cases would have been diverted from prosecution across the subsequent nine years if the diversion rate had stayed the same (“missed opportunities”). We multiplied the estimated number of these “missed opportunities” by the reduced frequency of repeat offences that the prosecuted offenders were likely to have committed, after adjusting for the time at risk by the number of years left in the study period. Then, based on a systematic review of diversion experiments (Petrosino et al 2010), we applied the standardized effect size of diversion in those studies to Farrington’s (1992) annualized crime frequency per 100 offenders aged 25, multiplying that effect across all of the person-years after a case was prosecuted rather than diverted, using both population-based rates and rates based only on detected offenders at that age.
Findings The diversion rate in Victoria dropped in half over ten years, from 25.6% to 12.5%. The total missed opportunities for diversion, compared to the counterfactual of applying diversion at a constant rate of 25% over that time period, totalled 115,885 cases over the ten years. Taking an average effect size (d = -0.232) across seven experiments with a mean followup time of 12-13 months, as derived from a systematic review of diversion experiment outcomes, our illustrative estimate is that at least 8 fewer crimes per year per 100 offenders could have been prevented among the missed opportunity cases. Using a population rate of offending, the estimate equals 1,474 crimes that could have been prevented. Using the offending population rate, we estimate that 37,050 offences could have been prevented.

Conclusions While the exact amount of crime prevented remains speculative, the application of best evidence to the missed opportunity cases suggests that more diversion could have resulted in substantially less repeat offending, and hence less total crime.