Re: st: Friedman tests

[Ron�n Conroy <rconroy@rcsi.ie>]

Ashley Harris wrote:

> Statalist,
>
> What I have nonparametric data for 3 groups, 1 dependent variable (ratio)
> and 2 independent variables (technique and analysis type).
Data are not parametric or nonparametric. And, indeed, the terms are 
confusing when applied to statistical procedures. I presume that you 
have a dependent variable that you can't put into a regression or anova. 
Nevertheless, before you head off an do a rank test, consider using some 
of the more powerful alternatives.

- counted data: consider negative binomial regression (or Poisson, if 
counts are of rare events)
- ordered categories: ordinal logistic regression springs to mind

With these techniques, you can explore multiple independent variables, 
just as you can in regression. Of course, these models have assumptions, 
just as any model does, and you should check that your data conform to 
these assumptions.

Rank tests tend to be an index of despair. They lose important 
information by losing the original measurement scale of the dependent 
variable. This allows you to assess statistical significance, but not 
real life importance.

--

Ronan M Conroy (rconroy@rcsi.ie) 
Senior Lecturer in Biostatistics 
Royal College of Surgeons 
Dublin 2, Ireland 
+353 1 402 2431 (fax 2764) 
-------------------- 
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