I fail to understand how Richard Butchart, a former head of statistics at the DfT, can claim that “for regression to the mean (RTM) to work” it is crucial that the mean be stable (Letters LTT 08 Jan). RTM, being a direct consequence of the immutable law of averages, always works and regression always occurs, towards whatever the mean level then is. In other words, RTM also includes Regression to Mean Trend. One graph of many confirming the point is for killed and serious injuries...
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