Sorry, I was just trying to make the point (badly) that they have said there is a point where we will see diminishing returns; that at N% of the population everything is fine but 100% is definitely not. In other words, somewhere between N% and 100% of the population we don't really want to add any more samples to the database (for various reasons). I was just wondering really if they have estimated whereabouts that point is.
It's an excellent question, and it's very likely that we're already at that point.
The "DNA detection rate" (i.e. the percentage of recorded crime which is detected, and where DNA evidence is available) has been within a whisker of 0.36% since 2002, even as almost a million innocent people's DNA has been added to the database. Adding all those innocent people's DNA has not made the database more effective at detecting crime, but it has cost money - so the cost-effectiveness of the database has actually been falling
since 2002 because of the expansion programme.
You can see the exact figures in this table from Genewatch:
http://www.genewatch.org/uploads/f03c6d ... NAFigs.pdf
For more background, read these excellent Genewatch reports:
"Would 114 murderers have walked away if innocent people’s records were removed from the National DNA Database?"
http://www.genewatch.org/uploads/f03c6d ... /brown.pdf
The DNA Expansion Programme: reporting real achievement?
http://www.genewatch.org/uploads/f03c6d ... _final.pdf
It's trivial to download the Home Office statistics on crime and DNA detection and do the arithmetic on the figures yourself:
Home Office crime statistics
http://www.homeoffice.gov.uk/rds/pdfs09 ... 009rev.xls
NPIA NDA database report 2009
The question is - if Genewatch can see this, and you and I can so easily repeat the calculations from published Home Office data, why can't the Home Office see it? They seem fixated on expanding the database in the teeth of the evidence that they're wasting taxpayers' money as well as encroaching on civil liberties.