Re: st: RE: Quantile question

[Ronan Conroy <rconroy@rcsi.ie>]

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?
Psychometrics

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
rconroy@rcsi.ie
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
http://www.flickr.com/photos/ronanconroy/sets/72157601895416740/

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