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PostPosted: Thu Aug 05, 2004 16:08 
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The Conservatives have announced that they are going to review camera locations, and scrap any cameras where an increase in accidents happened after their introduction. In principle, I think this could be a step in the right direction, but I am interested in how they will measure that.

I fear that if they just take a couple of years data, the decision could be affected by a ‘regression to the mean’ effect, whereby the increase in accidents could be due to statistical anomalies due to the random nature of small number statistics. It might be better to leave the cameras in situ and collect data over a longer term to avoid this effect. SafeSpeed has used the argument of ‘regression to the mean’ against speed cameras, but can the same argument be used to defer their removal?

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PostPosted: Thu Aug 05, 2004 19:03 
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I think it all depends on where the test statistic (KSI/fatalities/etc) originally lay in comparison to the mean value.

If the speed camera is removed when the test statistic was low in comparison to the mean, then regression to the mean will occur and will make it look like the removal of the camera has caused more accidents.

If the test statistic was high in comparison to the mean, then regression to the mean will happen in the opposite direction and it will appear that removal reduced accidents.

If the test statistic was near to the mean when the camera was removed, then taking an average for the next few years should hopefully provide a reasonable conclusion on whether or not that camera reduced, increased or had no bearing on accidents.

I hope that's all correct :).


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