Thursday, 14 January 2010

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!

Saturday, 2 January 2010

DDP Discussions and challenges

Several people have asked about benchmarks for DDP. I have actually blogged about this, but my comments are "buried" in the comments to the post about "Starting with DDP". Please have a look at the comments for that post, which include:

- benchmarking DDP with other organisations (raised by Bernhard Burger)

- Paul Herzlich's challenges about the seriousness of defects, getting data, DDP being hard to use and code-based metrics (all of which I have replied to in my following comment)

- using DDP to improve development (raised by Ann-Charlotte)

- Michael Bolton's challenges:
5 examples to show when it doesn't work (which I reply to in my first comment following his) (including some Dilbert Elbonian testers ;-)
7 "problems" - some of which I agree with, some I don't understand, and some I think illustrate the benefit of DDP rather than being problems with it (replied to in my second comment after his)

Thanks to Michael B's comments, I also formulated 3 Rules for when to use DDP:
DDP is appropriate to use when:
1) you keep track of defects found during testing
2) you keep track of defects found afterwards
3) there are a reasonable number of defects – for both 1) and 2)

These are not the whole story (as illustrated by Michael's examples) but I think are a pre-requisite to sensible use of DDP.