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Re: st: RE: Quantile question

From   Ronan Conroy <>
Subject   Re: st: RE: Quantile question
Date   Fri, 29 Feb 2008 22:23:25 +0000

On 29 Feb 2008, at 17:59, Nick Cox wrote:

It's not your question, but most classifications into quantile-based
groups sound perverse.
Why throw information away?

As predictor variables, quantiles have natural advantages over scores on psychometric instruments such as depression scales or aptitude scales. These advantages are

1. Easy to understand: the effect size is the effect of a 1-quantile increase in the predictor
2. Easy to compare: effect sizes for different predictors are comparable, allowing comparisons to be made
3. The quantiles are based on the actual distribution of the measure in the population, not on its theoretical score range. The adjust for the often far-from-nice distributions shown by psychometric measures.

In fact, where the predictor is measured on an unfamiliar and arbitrary scale, quantiles contain more information (in the hermeneutic rather than Shannonist sense) than the original scale values.

P Before printing, think about the environment
Ronan Conroy
Royal College of Surgeons in Ireland
Epidemiology Department,
120 St Stephen's Green, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95

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