With the help of statisticians from the University of Warwick, it has devised a formula for probation staff to work out whether or not someone is likely to reoffend.
It is this. Start with the number 31, subtract the age of the offender, subtract the number of previous youth custodies, then i, take total previous convictions; ii, calculate the number of years since the first conviction and add 5. Then divide i by ii, take the square root, multiply by 75, round to the nearest number and add to the score. Then add or subtract a number defined by the current offence.
The final score is then matched up to a percentage reconviction rate. Or put more simply: S = 31-A-C+ 75g/(F+5) + K.
The Offender Group Reconviction Scale, as it is known, is designed to help probation officers assess a criminal's risk to the public of reoffending when they draw up their pre-sentence reports for magistrates and judges and recommend community or custody penalties.
It was developed by Professor John Copas, head of statistics at Warwick, using information from a large sample of people listed on a Home Office offenders index. The key variables used to predict the probability of reconviction within two years were - age, sex, offence, the number of previous youth custody sentences ,the number of previous convictions and the rate of conviction.
Not surprisingly, perhaps, the formula has met with derision among those who are supposed to use it. They argue that, of course, if you are young, have masses of previous convictions and have ignored earlier custodial punishments, chances are you might offend again - you don't need a calculator and slide rule to work that one out. And if you have a history of violence then you clearly pose a greater risk to the public.
Further, they argue that experience and research have shown that key factors in recidivism are job prospects, education, stable homes and relationships, drug and alchohol abuse, criminal associates, mental health and rehabilitative work. It is not known whether these factors were considered.
Prof Copas was not available to expand on it but Harry Fletcher, assistant general secretary of the National Association of Probation Officers, said: "Probation staff have always believed that the chances of a person reoffending depend on factors like job prospects, education, and drug addiction - not square roots and algebra."
Leading article, page 12