The British Home Office want to keep a huge DNA database of people who've been acquitted of crimes (or arrested and then released with charges dropped), saying that "innocent people who have been arrested are as likely to commit crimes in the future as guilty people." In support of this "controversial assertion" they cite a piece of research that Guardian science columnist Ben Goldacre calls "possibly the most unclear and badly presented piece of research I have ever seen."
On page 30 they explain their methods, haphazardly, scattered about in the text. They describe some people "sampled on 1st June 2004, 1st June 2005 and 1st June 2006". These dates are never mentioned again. I have no idea what their plan was there. They then leap to talking about Table 2. This contains data on people each from a "sample" in 1996, 1995, and 1994, followed up for 30 months, 42 months, and 54 months respectively. Are these anything to do with the people from 2004, 2005, and 2006? I have no idea.
In fact I have no idea what "sample" means, perhaps that was the date they were first arrested. I don't know why they were only followed up for 30, 42, and 54 months, instead of all the way to 2009. Crucially I also don't know what the numbers in the table mean, because they don't explain this properly. I think it is the number of people, from the original group, who have subsequently been arrested again.
Anyway. Then they start to discuss the results from this table. They say that these figures show that arrested non-convicted people are the same as convicted people. There are no statistics conducted on these figures, so there is absolutely no indication of how wide the error margins are, and whether these are chance findings. To give you a hint about the impact of noise on their data, more people are subsequently re-arrested over the 42 month period than over the 54 month period, which seems surprising, given that the people in the 54 month group had a much longer period of time over which to get arrested.