Good answers above.
What bike you have, where you leave it, and for how long and how often, are all important factors.
A crappy looking unfashionable bike is less likely to be stolen than a flashy new one
A bike left in a highly visible spot, among other bikes, is less likely to be stolen.
A bike locked securely for two minutes is less likely to disappear than one left for two hours.
Leaving your commuter bike in the same place everyday is asking for trouble.
I have occasionally wondered if taking the chain off the chain ring, and letting it dangle, would improve security.