A New South Wales study has found longer prison sentences do not deter criminals from breaking the law.

The NSW Bureau of Crime Statistics examined trends in property and violent crime across the state between 1996 and 2008.

Bureau director Don Weatherburn says a 10 per cent increase in the risk of arrest or imprisonment had a small impact.

But the study found increasing jail sentences had no effect at all.

"The bottom line is this: increasing the risk of arrest and the likelihood of going to prison produces modest reductions in property and violent crime, but increasing the length of prison sentences exerts no effect at all," Dr Weatherburn said.

"The question most offenders are asking is, am I going to get caught? They're not sitting down and thinking, well if I am caught will I go to jail and if I do how long will I go for?

"The only question in the front of their minds, many of them not being great long-term planners, is what's the chance of being caught if I do this?"

Dr Weatherburn says the study found a booming economy is the surest way to reduce crime.

He says a 10 per cent increase in household income produced an estimated 19 per cent reduction in property crime over the long term and a 15 per cent cut in violent crime.