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Lords ‘paradox’ is a notoriously difficult puzzle that is guaranteed to provoke discussion, dissent and disagreement. Two statisticians analyse some observational data and come to radically different conclusions, each of which has acquired defenders over the years since Lord first proposed his puzzle in 1967. It features in the recent Book of Why by Pearl and McKenzie, who use it to demonstrate the power of Pearl’s causal calculus, obtaining a solution they claim is unambiguously right. They also claim that statisticians have failed to get to grips with causal questions for well over a century, in fact ever since Karl Pearson developed Galton’s idea of correlation and warned the scientific world that correlation is not causation.
However, only two years before Lord published his paradox John Nelder outlined a powerful causal calculus for analyzing designed experiments based on a careful distinction between block and treatment structure. This represents an important advance in formalizing the approach to analysing complex experiments that started with Fisher 100 years ago, when he proposed splitting variability using the square of the standard deviation, which he called the variance, continued with Yates and has been developed since the 1960s by Rosemary Bailey, amongst others. This tradition might be referred to as The Rothamsted School. It is fully implemented in Genstat® but, as far as I am aware, not in any other package.
With the help of Genstat®, I demonstrate how the Rothamsted School would approach Lord’s paradox and come to a solution that is not the same as the one reached by Pearl and McKenzie, although given certain strong but untestable assumptions it would reduce to it. I conclude that the statistical tradition may have more to offer in this respect than has been supposed.
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