Combining:
http://www.wired.com/news/politics/0,1283,66146,00.html
http://www.fcw.com/fcw/articles/2004/0705/web-finger-07-06-04.asp
We get some interesting numbers…
47,000,000 fingerprint records of ‘undesirables’ held by FBI
118,000 visitors to the US
99.6% match accuracy for two finger test against a single print (I’m going to assume false positive and false negative rates are the same since the stats don’t actually show that)
So, 4 people per 1,000 will be declared a match to each print, so on any particular day, matching against a single print there could be around 470 falsely declared matches (even assuming one is a correctly declared match – there won’t ever be more than one).
Now, multiplying this by 47,000,000 prints held suggests that with a fair degree of certainty every visitor to the US will match one or more of the ‘watchlist’ prints. Even with a watchlist of the order of 250 the birthday paradox is going to be kicking in and you’ll be seeing more people incorrectly declared matching than declared not matching on any particular day.
Which makes me wonder, how big is the watchlist right now? Surely there must be more than 250 people in the world who dangerously hate the US and are known to the US, but how do they get their prints? At what point is the whole exercise written off as ridiculous?