I've just used it in a discussion about hanging. Before WWII, the murder rate in the UK was around 8 per million (it's now up to around 14). Murder carried an automatic death sentence but the rate of hangings was around 13 a year showing that most were commuted to life sentences. Given that the population was around 40 million, the hanging rate was 0.3 per million.
It wasn't a discussion about the deterrent effect although I do tend to agree that capital punishment deters one person from committing more crime.