WALK down Evington Road towards Leicester station and you pass chicken shops, betting parlours and, more likely than not, a group of men drinking lustily on the corner. Police used to visit the neighbourhood only once every couple of days. But now an officer is a permanent fixture. Even if trouble starts up elsewhere in the Stoneygate area, she will stick to her beat. This is one of the first changes brought about by an experiment in the way that police count crimes, which could radically transform the way they work.

Six months ago Leicestershire Police began to put the Cambridge Crime Harm Index (CHI) into practice, one of at least five forces to do so. Previously all crimes had been counted equally in the annual tally, with murder and petrol-theft both being given a value of one, among more than 60,000 offences. Lawrence Sherman, a professor of criminology at Cambridge University who designed the CHI, believes that this form of statistical equality can mislead the public and encourage police to focus on the easier offences. A big increase in murders, say, could be cancelled out by a small drop in shoplifting. But multiply the number of times a crime occurs by the harm it causes its victim and the statistics tell a clearer story.

How to measure harm? It is a controversial business. The CHI takes the official guidelines on the minimum recommended sentences for each crime as its proxy. Shoplift, and the basic sentence is two days in prison; nobody has been hurt very much. Commit a murder and the least time you can expect to spend behind bars is 5,475 days. The Office for National Statistics is about to publish its own crime-harm index for all 43 police forces in England and Wales, based on an average of the sentences actually handed out by judges over the past five years. Professor Sherman points out that this varies according to the criminal record of the culprit, which has no bearing on the victim.

When viewed through the CHI, some places in Leicestershire that were previously considered relatively low in crime turn out to be worse than thought, and vice versa (see maps). Spinney Hills, in the centre of Leicester, ranked 13th out of 67 areas in terms of the volume of crime. But it comes fourth in terms of the harm caused. Within it, a small section of Evington Road, near the booze and bargain stores, raised a red flag in the new system: two rapes and a few cases of grievous bodily harm had occurred there over the previous three years.

The officer who now patrols only in that postcode has been trained to micromanage tricky areas, as have 70 others on the Leicestershire force who now have harm-specific roles. She aims to get to know the few Evington residents at the sharp end of serious crime (80% of harm in Leicestershire is suffered by 5.6% of victims, and caused by 4.7% of offenders, police say) and connect them to other services, such as Turning Point, a drug-and-alcohol rehabilitation charity, and domestic-violence units. Her work is narrower, more proactive and far more joined up than before, says Phil Kay, Leicestershire’s assistant chief constable.

Priorities are changing at the top, too. Looked at by volume, sexual offences constitute less than 2% of Leicestershire’s total. In terms of the harm caused, that number is 30%. Police already knew such crimes were important, says Mr Kay, but the statistics make a powerful case to do more. Seventy-seven more officers have been moved to work on sexual-violence cases.

The indices will need fine-tuning. It makes more sense to record crime harm from the date it occurred, rather than when it was reported—otherwise the recent surge in reports of historical child sex-abuse, for example, could give the false impression of a big recent increase in crime.

Some forces will lack Leicestershire’s chutzpah in using the data. And harm-indices may encourage police to devote less time to fixing broken windows and other minor complaints which, some criminologists believe, are the key to preventing more serious crimes. But with the state-funded College of Policing pushing hard for evidence-based practice and resources stretched by the loss of 14% of officers since 2010, there is momentum. Mr Kay is blunt: he joined the police to stop people being stabbed, not petrol being stolen.