How unusual are rape data "inconsistencies"?

The reputation of crime statistics has taken a few knocks of late, after a committee of MPs heard evidence that figures were often deliberately manipulated to under-report certain crimes.

Rape statistics were today under the microscope, with campaigners claiming that data showing variation in the proportion of rape allegations that different police forces pursued was evidence that there was a "culture of disbelief" in some areas.

The figures themselves were published this morning by HM Inspectorate of Constabulary (HMIC) on behalf of the Rape Monitoring Group - a joint initiative by senior police officers, Home Office and Ministry of Justice officials and crown prosectutors to identify areas for improvement in how rape cases are handled.

While much of the data isn't new, it has attracted a lot of media attention, particularly when it comes to the "no-crime" rate in different police force areas. This rate shows the proportion of rape allegations which were dropped by police because officers felt there was insufficient evidence that a crime actually took place.

While a third of the allegations of adult rape brought to the attention of police in Lincolnshire were later dropped, the same was true of just 3% of adult rape allegations in Cumbria and South Wales.

So is this level of inter-force variation unusual? The Home Office actually publishes force-by-force breakdowns of the no-crime rate for a variety of different offences, which can give us some context to this question.

It isn't uncommon for there to be quite a wide divergence in the no-crime rate for certain offences. For example, West Yorkshire Police dropped 24% of alleged weapon possession cases, while Essex dropped just 1%. However no other offence group detailed in these statistics had quite as wide a range in the 'no-crime' rate than rape.

Whether or not this is evidence of a "postcode lottery", as some campaigners have suggested, or weaknesses in the data is more difficult to answer. Some media reports have noted that the Metropolitan Police - which undertook an internal review on its handling of rape allegations in 2009 - has seen its no-crime rate almost halve from 24% in 2008/09 to 13% in 2012/13.

However rape was also one of the areas where the Metropolitan Police Service has been accused of under-reporting by reclassifying alleged crimes as "crime related incidents", and a report by the Bureau of Investigative Journalism concluded earlier this month that once these were factored in "nothing changed" following the Met's review.

Rape statistics, as we've seen before, need to be particularly carefully understood, and the figures released today may tell us as much about how different police forces handle data collection as it does about how they handle rape allegations.