If you love statistics, modern government must be heaven. Whether it’s due to the complexity of big government, the realities of modern life or the control freakery of New Labour, or some mix of all three, you can barely breathe in most policy areas without inhaling a lung full of statistics, normally sprinkled with a few decimal places for that added dash of veracity.

The area around crime is particularly prone to not just statistics, but arguments over statistics. So I find I always have to tread with care through all the numbers to try to pull out the essence of what’s going on.

One pair of numbers I have put together recently gets to the heart of much of the debate. It is this: the cost of jailing someone for one year is the same as the cost of employing a full time police officer for a year.

There are allowances you have to make for capital versus running costs and so on, but basically one extra person in prison equals one less police officer.

So when Tories and Labour thump their tubs about being tough on crime, we shouldn’t feel meek about pointing out the costs of their failed policies. Having a large prison population is not a sign of success – it is a sign of a failure to prevent crime and it leaches huge resources away from other parts of the justice system. Instead of prioritising preventative police work and rehabilitating re-offenders, money is sucked into cramming people into poor conditions.

Now – you might say, they are criminals so who cares how bad their conditions are? This is where the difference between those who emphasise the vindictive part course from those who emphasise the effective. For – whilst there are some people who should be locked up for life – nearly everyone else will leave jail at some point. Even if we abolish the controversial automatic reductions in jail sentences for good behaviour (which the Liberal Democrats opposed in Parliament) – the overwhelming proportion of prisoners will still be released at some point.

To me, a priority is to cut the crime rate amongst released prisoners. For if we can do that – then we can stop people becoming victims of crime. And it’s not just a few future crimes we can prevent – because re-offending rates for people released from prison are shockingly high. Just under six in ten of the prisoners released in 2003 were caught committing at least one offence in the subsequent two years. Amongst those who stole from vehicles, the re-offending rate was just under nine in ten. Clearly prison isn’t working for such offenders – which is why we need to make it work.

Prison certainly has its place – and there are those who should be locked up, and locked up for a long time. Longer jail sentences for some crimes (such as knife crimes, as we proposed in Parliament – but were blocked by Labour) have their place too. But prison isn’t the whole answer. If it was, we – with our massive and soaring prison population – would have plummeting crime rates. We don’t.

The alternative – as Alan Sherwell, the Lib Dem leader in Aylesbury Vale, put it well in Liberator no.314 – is to emphasise rehabilitation, re-education, offering people with mental health problems proper treatment and tackling drug addition.

We can’t go back in time and undo a crime a criminal has committed, but if we stop them re-offending – then we have zipped forward in time and stopped a crime before it occurs. If it is a choice between being vindictive about the past or effective in cutting crime in the future, my vote is for being effective.

So when Labour or the Tories attack us on crime, there is no need to be defensive or on the back foot. We should be on the front foot – harrying them over why they want to cram more and more people in bad jail conditions with the result that jail simply becomes a pause for rest between one crime and another. And all the while this failure to cut crime is taking resources away – at the cost of one police officer for every extra jail place needed – from the police and other parts of the justice system.

The same front-foot mentality should also apply to issues around civil liberties – pointing out that plans for ID cards etc mean putting huge amounts of money and effort into keeping tabs on innocent people. Wouldn’t it be better to concentrate on criminals and terrorists instead?

With the Criminal Records Bureau in such a mess, and all the problems we’ve seen this year keeping track of people who should be having deportation cases heard, simply keeping tabs on such people is clearly more than enough for the Government to struggle with, without adding in tens of millions of innocent people too.

It’s effective police focus on the criminals, not the innocent, that we need and should argue for with pride.

This article first appeared in Liberal Democrat News. For subscription details, click here.