There are plenty of obvious differences between Hertfordshire and Dorset. One is landlocked; the other boasts Lulworth Cove. One buzzes with human and commercial activity along the M1 and A1 corridors; the other with the teeming insect life of ancient hedgerows. One nurtured Shaw; the other Hardy. But none of these explains the difference on which we report today: drugs cases are eight times more likely to end in acquittal in Hertfordshire than in Dorset.

The reasons are complex and not always readily understood. They involve the varying abilities of police and prosecutors, the often conflicting guidelines for pressing criminal charges, and the texture of society itself. Juries in St Albans and Bournemouth, considering the same evidence, might simply disagree.

The same factors have produced striking anomalies across Britain’s judicial map. In Northumbria, nine out of ten alleged public order offences lead to convictions. In Bedfordshire, the figure is barely six out of ten, with 47 per cent of cases of alleged assault being thrown out or ending in acquittal. In Merseyside, the Crown Prosecution Service (CPS) won convictions in 84 per cent of murder trials between September 2004 and August this year. But in London the figure was just 66 per cent, and in Sussex and Stafford it was even lower.

The existence of broadly comparable data for the CPS’s 42 regional units is a step forward for a service whose performance has hitherto been notoriously hard to measure. But the fact that The Times was forced to use the Freedom of Information Act to obtain the data is lamentable considering the stated aim of the Director of Public Prosecutions of making the CPS more open and accountable. And the conclusion to be drawn is troubling: for all the CPS’s laudable efforts to relieve police of the burden of deciding when to press charges in criminal cases, its delivery of justice is unpredictable and inconsistent.

In the 11 months under review, 41,000 criminal cases were pursued by the CPS to the eve of trial only to be dropped for lack of evidence. Of these, 7,000 were in London. Inevitably, a proportion of cases “crack” as trial dates approach, but, unfortunately, some cases proceed too far, at great expense, before being properly reviewed by the lawyer assigned to prosecute them. This is hardly surprising given the burden on CPS lawyers, but it remains true that earlier, more rigorous screening for cases that will not stand up in court would free up resources for the viable — and for the severely constrained legal aid budget for defendants.

Instead of such screening, the CPS has introduced an all-hours telephone hotline. In principle this gives police instant access to lawyers who decide, often from home and at short notice, whether to press charges. In practice, those lawyers are often forced to observe the mantra “if in doubt, charge”. The result has been a new surge in cases doomed to fail. Likewise, new powers to arrest and charge those suspected of sexual assault without the co-operation of victims, though born of the best of intentions, can lead to unwinnable trials.

There are no easy routes to securing more convictions while keeping the scales of justice balanced. But greater transparency will at least reveal the hard choices to be made.