Re: st: Modeling % data

[Ronan Conroy <rconroy@rcsi.ie>]

On 22 MFómh 2010, at 16:03, Marlis Gonzalez Fernandez wrote:

> My outcome variable is a % (% error in a language test).  We do have  
> many 0 and 100.  I need to be able to do a multiple regression to  
> adjust for known predictors of the variable vs. the predictors of  
> interest.

These test floor and test ceiling effects occur when the person's  
ability is either above or below the detection range of the test. I  
suggest that if the scores are normalish in the observed range, you  
use -intreg- and treat the zeros or hundreds as ≤0 and ≥100. The  
problem is that these scores are not necessarily true zeros (no  
ability) or true 100% (perfect language). They reflect the range of  
abilities spanned by the test. Treating them as zeros and hundreds  
will bias the estimation of differences between groups.



Ronán Conroy
Associate Professor
Division of Population Health Sciences
=================================

rconroy@rcsi.ie
Royal College of Surgeons in Ireland
Epidemiology Department,
Beaux Lane House, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
+353 (0)1 402 2764 (Fax - remember them?)
http://rcsi.academia.edu/RonanConroy

P    Before printing, think about the environment





*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/