Speed cameras, regression-to-the-mean, and the Daily Mail (again)
It was interesting to hear ‘regression-to-the-mean’ being discussed on the Today programme this morning, even if the quality of the debate wasn’t great. The issue was the effectiveness of speed cameras, which tend to get installed after a spate of accidents. Since bad luck does not last, accidents tend to fall after such a ‘blip’, and this fall is generally attributed to the speed camera, whereas it would have happened anyway: this is what is meant by ‘regression-to-the-mean’.
The report from the RAC Foundation tried to deal with this by essentially ignoring the 3 years before the camera was installed, and so comparing the post-installation accidents with those more than 3 years beforehand, and simultaneously allowing for overall changes in accidents over time. Unfortunately the report is not very clearly written, more discussing how to approach and analyse the (limited) data than aiming to provide definitive results. Although they helpfully provide the equations for the models being fitted, there is no executive summary and you have to search quite hard to find the crucial number flagged up for the media: the estimated 27% reduction in accidents causing fatal or severe injuries (page 32).
I thought the analysis seemed quite reasonable until I noticed that on page 3 it defines a baseline year as
‘more than three full years before the camera was established or the year during which it was established’
It seems very strange to include the transitional year as a baseline – surely it could just be excluded? Later on the report says that if the start-months were January or December, the year in which the camera was installed was treated as a ‘camera’ or ‘within 3-year pre-camera’ year respectively, but I am suspicious that for the remaining 10 months this could mean that some random-high accident rates could still be included in the baseline.
However, what is really shocking is the grossly misleading coverage of the Daily Mail, with the headline,
This is a blatant mis-representation of the report and its findings, focusing solely on the 21 cameras where an increase was estimated, and ignoring the 530 where it wasn’t, as clearly shown in the table the Daily Mail so helpfully reproduce! They should be ashamed of themselves.