The Effects of Sanction Intensity on Criminal Conduct - JDAI Helpdesk

statistical model that would predict the risk <strong>of</strong> serious re<strong>of</strong>fending 1 so that the whole
APPD population could be stratified by risk level.
<strong>The</strong> risk prediction model
<strong>The</strong> statistical model used to forecast the risk <strong>of</strong> serious <strong>of</strong>fending is described in
full in Berk et al. (2009). Random forests methods were applied to a dataset <strong>of</strong> all
probation and parole cases in Philadelphia between 2002 and 2004, containing only the
data available to probation <strong>of</strong>ficers at intake, 2 to predict the risk <strong>of</strong> being charged with a
new serious crime within two years <strong>of</strong> the probation or parole case start date. Random
forests is a statistical learning procedure that forecasts outcomes by aggregating results
from multiple classification and regression trees. <strong>The</strong> model was designed to stratify the
population according to APPD’s operational needs, with the assumption that the majority
<strong>of</strong> the caseload was at low risk <strong>of</strong> serious recidivism and thus appropriate for lowintensity
supervision. At the agency’s request, 61 per cent <strong>of</strong> cases were to be deemed
low risk, with the remainder either high risk (approximately 10 per cent) or neither low
nor high (approximately 30 per cent) (Fig. 3.1). APPD also deemed the proportions <strong>of</strong>
false positives and false negatives expected in the final model to be operationally
acceptable. <strong>The</strong> proportion <strong>of</strong> false positives (<strong>of</strong>fenders erroneously identified as lowrisk)
was set at 5 per cent, and the proportion <strong>of</strong> false negatives (<strong>of</strong>fenders erroneously
identified as high risk) was 20 per cent. A higher false negative rate was accepted given
the lesser public safety concerns around this type <strong>of</strong> error. <strong>The</strong> initial 2002-2004
probation dataset is described as a “training sample,” which is used to ensure the
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