How to Count Crime: the Cambridge Harm Index Consensus
By Lawrence Sherman and Cambridge University associates
Crime statistics require a radical transformation if they are to provide transparent information for the general public, as well as police operational decision-making. This statement provides a blueprint for such a transformation.
Summary
The best way to count crime is to assign a weight to the harm caused by each crime, rather than by counting all crimes as if they were created equal. This can be done by summing up the days of imprisonment recommended by sentencing guidelines for each crime type, multiplied by the number of crimes of each type that were reported by victims or witnesses, then summing the weight across all crime types to equal total crime harm. Total harm can also be calibrated by any other democratically legitimate method of assigning harm levels for each crime category in relation to all others. Any method using sentencing guidelines based on the harm caused by an offence to victims, without regard to the offender’s prior record or other circumstances, meets the standard of the Cambridge Crime Harm Index (Sherman et al. 2016).Footnote1 Other methods of weighing harm for each crime can apply the same principles outlined in this statement.
By using a single sum of weighted crime harm across all crime types, for each year in each community, governments can offer the public a more reliable indicator of their safety. The Crime Harm Index (CHI) would also provide a clearer indicator by leaving out crimes not reported by the public at large, such as police-initiated investigations of human trafficking and narcotics crimes, as well as crimes such as big-store shop-theft that are detected by a company’s security staff. The clarity comes by separate reporting of proactive investigations that measure of varying levels of investment in detections rather than actual crime levels. A CHI also leaves out crimes reported in the current year but which occurred in a prior year—because the existing system of counting crimes when they are reported but years after they occurred distorts the measurement of current public safety for which police are held accountable.
The single CHI sum also lends clarity to other relevant indicators, such as the “detection” rate, which currently treats all crimes as created equal. It allows police to invest scarce resources in proportion to the harm of each offence type, by showing the public the proportion of all harm for which police bring offenders to justice. A Harm Detection Fraction (HDF) would use CHI to give the fact that over two-thirds (67%) of murders are detected far more weight than the low detection rate for vandalism (Office of National Statistics 2019). Similarly, a Proactive Policing Index (PPI) would use the CHI to give credit—instead of blame—to police for detecting hidden slavery and organized crime. The annual national and local crime reports recommended by this Consensus Statement are comprised of these seven statistical series to be calculated consistently from each year to the next:
A Crime Harm Index (CHI) for crimes against victims in the current year.
Crime counts by all crime categories, used to calculate the CHI.
A Historic Offences Index (HOCHI), a CHI for crime occurring in prior years.
A Proactive Policing Index (PPI), weighted by crime type as for the CHI.
A Company-Detected Crime Harm Index (CDCHI), also weighted by CHI.
A Harm Detection Fraction (HDF), which is the proportion of CHI with police detections.
Detection rates per 100 by all crime categories, used to calculate the HDF.
This system would give the public a reliable and realistic assessment of trends, patterns and differences in public safety. It would also give police a proportionate system of incentives to manage demands for their services with a clear focus on cutting the harm from crime, and not just the high volume of low-harm crimes counted equally. It offers a “bottom-line” for crime, like the profits of a business: a clear metric that untangles the current confusion about what the profusion of crime statistics really means for the general public.
Camb J Evid Based Polic 4, 1–14 (2020).