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There's popular support for locking up those found guilty of knife crime. That's particularly in west central Scotland, where chib culture has an unusually strong grip on young men.

Both Labour and Tories back versions of that policy, with at least six month mandatory sentences.

There's also popular support for ending the automatic, unconditional early release - at the half-way point in their sentence - of prisoners serving less than four years.

The four main parties in the Scottish election are agreed on that much, despite the fact that all four of them have, at different times, operated the policy while in government and in charge of the justice system over the past 18 years.

Then there's a move to bring back the handing down of short prison sentences. Labour and Conservatives favour that, while SNP and LibDems want more non-custodial sentencing for less serious crimes.

Three policies. All quite popular in their own way. Not much evidence that they have a deterrent effect. And rather expensive.

Over-crowding

As part of ³ÉÈË¿ìÊÖ Scotland's reality check on election issues, I've been looking at the figures on this.

Let's start with the state of prisons now. With a bit of rounding off, the prison population has risen from just over 6,000 ten years ago to 8,000 now. The crime rate has been falling, for a number of reasons, but some more serious crimes are on the rise, and average sentences have been getting longer. The expectation is that the prison population will continue to rise, without any policy changes being made.

That puts pressure on prisons. There's currently design capacity for around 7,300 inmates. The new Low Moss prison in East Dunbartonshire is due to open next year, taking that up to 8,000 - by which time the prison population will probably be north of that figure. So there's over-crowding already.

Meanwhile, resources are tight, to say the least. In the new financial year, the Scottish Prison Service is taking a 24% budget cut, much of that in its capital budget, which is down 66%.

That's one of the biggest cuts of any Holyrood budget, and provides a tough financial context for all these sentencing policies.

Deterrent effect

The official reckoning on the impact of ending automatic early release of prisoners is that it would require between 700 and 1,100 places. That explains why the legislation was passed back in 2006 to reverse the policy, but it's never been implemented.

The cost of jailing everyone convicted of knife crime is quite a bit more. Official statistics show those convicted in the most recent annual figures stood at 3898, which was a significant fall on previous years.

Roughly a third of those convicted already go to jail. So if you're going to jail the others for an average of six months, that's around another 1,300 prison places required.

Supporters of the policy suggest there could be much more rounding down once you take account of the deterrent effect - that is, the fear of prison will keep people from carrying knives.

But senior police officers say that deterrent doesn't work, and we've heard from other experts in the field who agree. Even the experience of being in jail or young offenders institution doesn't seem much of a deterrent, given that well over half go on to re-offend within two years.

Incarceration

So let's put that together. There's no spare capacity in prisons, and budgets are being sharply cut.

Between an end to automatic early release and the start to automatic jail terms for those carrying knives, it looks like 2,200 extra places.

There's a new prison planned in Grampian to replace Peterhead and Aberdeen jails. Although there's no money available for it - or any other replacements of older prisons, including Inverness - HMP Grampian's budget is at least £100m for 550 places.

If that's the going rate for building prisons, the bill for these policies looks like a capital cost of around £400m. That's before running costs, when the price of incarceration runs at more than £30,000 per head per year.

That's before you calculate the cost of reinstating short sentences, as it's hard to guage how that might work and how extensively it would be applied by courts.

Even without that element, it doesn't look like the manifesto pledges being made on crime will be possible to deliver when budgets are so tight.