3. Comparing reconviction rates across local authorities
Estimating reconviction rates for local authorities
In Reconviction rates in Scotland, we have previously only published reconviction rates for local authorities based on information for offenders convicted in courts that fall within that local authority area’s boundary. This is because it was the only information on local authority that we could obtain. However, the areas that courts serve don’t exactly match local authorities; and offenders may be convicted in a court located in a different administrative area to where they live, yet they would be supervised in their area of residence (see Annex A11 and the footnote of Table 11). The characteristics of offenders are also likely to vary across local authority areas, therefore such comparisons between areas should be treated with caution, and it is suggested that a method which takes these factors into account should be employed (see section 3.1).
To improve estimates of reconviction rates for local authorities, we have started to collect data on the first half of an offender’s home postcode from Police Scotland, for example EH1 or G1. This data can then be used to match an offender to their home local authority. This information will be particularly important for local authorities who use these statistics for planning purposes, such as schemes to reduce reoffending, or estimating the number of offenders that social workers need to supervise in their area. Local authority reconviction rates based on offender postcodes are published for the first time this year, but due to incomplete postcode coverage we will still publish reconviction rates based on court area until it improves. We recommend that the figures based on court area are still used as the definitive local authority reconviction rates.
Local authority reconviction rates based on court area
Reconviction rates vary across local authority groups (based on the area covered by courts). Note that because some sheriff courts cover more than one local authority, we cannot distinguish between convictions in the different local authorities so some local authorities are grouped together, so that there are 25 groups rather than 32 separate local authorities. Table 11 shows that the highest reconviction rate in the 2015-16 cohort was for offenders whose index conviction was given in courts in Ayrshire, East, North and South (30.1%). Aberdeen and Aberdeenshire had the highest number of reconvictions per offender on average (0.56). The lowest reconviction rate (13.1%), and lowest average number of reconvictions per offender (0.19), were for offenders whose index conviction was given at a court in Na h-Eileanan Siar. These are unadjusted figures which do not take account of underlying differences in population size and the characteristics of offenders in each area (see section 3.1 for comparisons of standardised reconviction rates which take these into account).
Reconvictions tend to fluctuate year to year for local authorities. Smaller local authorities tend to have larger fluctuations as they have small numbers of offenders. Small between year fluctuations in the numbers of offenders reconvicted may lead to larger changes in the reconvictions in percentage terms than for local authorities with larger numbers of offenders.
Local authority reconviction rates based on residence
For the first time, we have published reconviction rates based on the local authority of offenders’ residences, by matching the local authority to the first half of offenders’ postcodes. These are presented in Table 12. The local authority reconviction rates based on postcode are currently labelled as Experimental Statistics: Data being developed, as we did not have postcodes for almost a quarter of offenders (24%) with an index conviction in 2015-16. Postcodes may be missing because offenders have no fixed abode, but it may also be a recording issue. This missing total also includes a small number of cases where postcode information was recorded, but it wasn’t a valid Scottish postcode.
Custodial sentences have a higher percentage of missing postcodes compared to other sentences. 36% of offenders with an index disposal of a custodial sentence were missing postcodes. This isn’t surprising as many custodial sentences would have started before Police Scotland started sending us conviction data with postcode information. Sentences over 4 years have the highest percentage of missing postcodes, with 55% of offenders missing postcodes. Short sentences also have relatively high percentages of missing postcodes, with 39% of those given sentences of less than three months missing postcodes, and 38% of those given sentences of 3 to 6 months. For offenders with sentences of 6 months to 4 years, about 30% were missing postcodes. Note that the data quality issues around the recording of postcodes only affects the local authority reconviction rates presented in Table 12 and does not affect any of the other reconviction rates presented in this publication.
Annex Table B1 shows the number and percentage of offenders with missing postcodes in each local authority group, based on the location of the court they were convicted in. It also shows the percentage of offenders living in the local authorities that are covered by the court areas, and the percentages that live in different local authorities to those covered by the court areas. This shows that there are significant percentages of offenders who are convicted in a court that covers a different local authority to where they live. For example, for offenders convicted in courts in Aberdeen and Aberdeenshire in 2015-16, 56% of offenders were known to reside in Aberdeen and 27% resided in Aberdeenshire, but 6% resided in another local authority, and 10% had missing postcode data.
The group with missing postcodes had a higher reconviction rate and average number of reconvictions (31.4% and 0.58) compared to the national rates for Scotland as a whole (27.0% and 0.47). This suggests that there is possibly some bias in the recording of postcodes. This could potentially be explained by this group having a higher proportion of offenders with short term custodial sentences that have high reconviction rates. About 15% of the offenders with missing postcodes (1649) had short term custodial sentences of under a year, which may slightly inflate the reconviction rate for the group with missing postcodes, but doesn’t fully explain the higher reconviction rates for group with missing postcodes.
Reconviction rates for local authorities based on the postcode data were generally lower than those based on court area, due to the potential biases in the recording of postcodes as discussed above. Therefore the actual reconviction rates for each local authority are potentially slightly higher than presented in the table. Reconviction rates based on postcode data varied between 31.8% for Inverclyde to 11.9% for Na h-Eileanan Siar (Table 12). However, as Annex Table B1 shows, there is variation in the percentages of missing postcodes between local authorities, so this direct comparisons between local authorities should be treated with caution. Also, different local authorities may have different mixes of offender characteristics, and small local authorities may experience greater fluctuations, which should be considered when comparing local authorities. The next section discusses these considerations in more detail (although those comparisons of local authorities are based on court area, the same factors would apply here).
3.1 Accounting for the variability between local authorities
Reconviction rates could be used to rank performance across different local authorities. However, there is an inherent problem in using this approach since it implicitly assumes that a difference in reconviction rates reflects a ‘real’ difference between local authorities. In reality, all systems within which these local authorities operate, no matter how stable, will produce variable outcomes in the normal run of events. In particular, outcomes in local authorities with smaller sized populations tend to vary more than those in local authorities with larger populations. The question we need to answer is therefore: Is the observed variation more or less than we would normally expect?
In this respect, it is better to use a method of comparison that takes account of inherent variability between local authorities. The funnel plot is a simple statistical method that takes into account the variability of different sized populations and so highlights whether there are differences that may be attributed to some other special cause.
Table 11 shows the average number of reconvictions per offender and reconviction rates for each local authority group (based on court area of conviction) and Chart 10 shows these reconviction rates against the number of offenders. The plot takes into account the increased variability of the local authority groups with smaller populations, where a small increase in the number of reconvictions may lead to a large percentage change in the reconviction rate. Rates for local authority groups which lie inside the funnel are not significantly different from the national rate, and we can then usefully focus on possible explanations for rates which deviate significantly from the national figure. In this case, the cut-off level for statistical significance is 95% (or two standard deviations from the mean): if there were no difference between local authorities groups apart from that which could reasonably be attributed to random variation, we would expect that 5% of the authorities (i.e. only 1 of them) would lie outside the funnel.
Chart 10 shows that East, North, and South Ayrshire and North and South Lanarkshire lie above the funnel, and so have a higher reconviction rate than expected. Argyll and Bute, Edinburgh and Midlothian, Na h-Eileanan Siar, Moray, Orkney Islands, Perth and Kinross, Shetland Islands, and West Lothian lie below the funnel and so have lower rates than expected. Whilst this is useful for highlighting that there are practical differences in reconviction rates between each local authority group, even after taking into account differences in population sizes, it does not allow us to identify if this disparity is due to variation in the characteristics of offenders in each area or a variation in practices between different local authority groups. Different offender characteristics between local authority groups could include: age, gender, crime, disposal, ethnicity, deprivation, etc.
Chart 11 is standardised to take into account some of the differences between local authority groups attributable to the characteristics of offenders, such as the number of previous offences, sentence, gender, and age. It provides the standardised reconviction rates against the observed number of offenders minus expected number of offenders. Since all local authorities groups are within the funnel it suggests that the apparent differences in reconviction rates in Chart 10 are primarily attributable to either the variation in the characteristics of the offenders, the type of crime they committed, or the sentence they received, rather than differences in ‘performance’ between the local authority groups. This overall conclusion for all local authorities on the 2015-16 cohort is consistent with previous findings in the Reconviction Rates in Scotland publications.
Chart 10: Reconviction rates by local authority group: 2015-16 cohort
Chart 11: Standardised reconviction rates by local authority group: 2015-16 cohort