Here is a paper that got a lot of press coverage. At first glance it appears to show that an intervention designed, among other things, to reduce teenage pregnancy had the opposite effect. With hindsight it seems obvious: bring together a bunch of rebellious teenagers and they'll end up having sex; what's more it'll be unprotected sex.
An accompanying editorial provides some context. Britain had the highest teenage pregnancy in Europe; a programme in America seemed to reduce teenage pregnancy by half; who wouldn't want to try it over here? Two problems with this.
Firstly what is the 'it' we want to try? What exactly do we replicate? Trial programmes are run by enthusiasts; results always degrade with wide spread (in our case nation-wide) implementation. You can't replicate enthusiasm.
Secondly the headline result from the USA provided only a partial picture from six of 12 pilot sites. Results from all 12 sites showed no benefit.
Even so, reversal of effect is unexpected, and we must congratulate the Department of Health policy research team for commissioning this evaluation.
Meanwhile over at the Times newspaper, an editorial ponders the difficulty of basing policy on evidence. It's not as easy as you think.