Evaluating the Death Penalty

Richard Ingrams, in his blog for The Independent, wonders how many innocent people would have been executed if capital punishment had not been abolished. It is a valid point, but not a balanced one. To evaluate the issue fairly, we need to know how many innocent people would have been murdered if capital punishment had been retained.
Statistics are helpful. According to the Home Office, between 1965 and 1998 71 people were murdered by people who had been released after serving 'life sentences'. That number alone probably dwarfs the number  of people wrongly found guilty; what we cannot know, however, is how many people were deterred from committing murder by the presence of the death penalty.
The question, pace Mr Ingrams, is still very much open.