Finding 40% of your own mistakes

I used to be fond of quoting a statistic that says you can only find around 40% of your own mistakes.

Michael Stahl emailed me to ask where this number came from. Interesting question - first thought - I don't remember! I'm sure I must have read it somewhere at some time, but where, by whom and was it based in a study?

I checked with Mark Fewster, one of my former colleagues, and he thinks it might have come from a study done by the Open University in the UK.

I checked with Tom Gilb, as he uses an estimate of around a third (33%) for the effectiveness of an initial inspection - which is probably more effective than an individual anyway! Tom has demonstrated an effectiveness of 33% repeatedly by experimentation with early Inspections; he said it also agrees with Capers Jones' data.

I think we used the figure of 40% only because people found it more believable than 33%.

The frightening consequence is that if you don't have anyone else review your work, you are guaranteed to leave in two thirds of your own mistakes!