My latest Guardian column "Surveillance: You can know too much," explains how collecting too much information on innocent people makes it harder to catch guilty ones:
At a certain point, data gathered to predict the weather overwhelms your capacity to add it to your calculations efficiently, resulting in ever-longer runtimes that give less accurate predictions. It's better to crunch the data needed to calculate tomorrow's weather in 10 minutes (and refine your guess twice an hour) than to shovel so much data into the hopper that you don't get tomorrow's forecast until next week.
The sweet spot lies somewhere between gathering too much information and gathering too little – and the secret to hitting that spot is intelligent, discriminating data-acquisition.
Take London: cover every square inch of the city with CCTVs and you'll get so much information that you'll never make any sense of it. Scotland Yard says that CCTVs help solve fewer than 3% of all crimes, while a study in San Francisco found that at best, criminals simply move out of camera range, while at worst they assume no one is watching.
Similarly, if you take fingerprints from every person who applies for a visa – or worse still, from every person in Britain who has to carry one of the proposed new biometric cards – you will fill the databases with chaff that slows down searches, generates endless false matches, and threatens everyone in the database with the worst kind of identity theft.