“How long does it take the health care system to kill 500 people?” asked Ross Wilson at the Quality Forum in Berlin two weeks ago (you can watch the video of his talk, and others, at http://internationalforum.bmj.com). His point was that we just don’t know. The global airline industry is able to say how many passengers die each year (500 in 2007) and why. By comparison, health care is flying blind.

Part of the problem is that we can’t agree on which data to collect or how to interpret them. This week Mohammed Mohammed and colleagues (doi:10.1136/bmj.b780) report their …