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Proc Genmod - Convergence Issue

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Proc Genmod - Convergence Issue

Dear all,

I am learning to use proc genmod. I am running an analysis of quality of life data: around 400 subjects, continuous outcome and scores range from -0.24 to 1.00. The distribution of scores is strongly left skewed. My model include 1 continuous predictor and 5 indicator variables. For this reason OLS assumptions are not met. Raising the 3rd power of the scores would allow meeting regression assumptions. I also linearly transformed the distribution of scores to have only positive values with the following:

QOL1=(QOL+0.25)*100

Following is the Genmod code used for the analysis.

Proc genmod data=[data-set]
class v1 v2 v3 v4 v5
model y= cov1 v1 v2 v3 v4 v5\ dist=norm link=power(3);
run;

the model did not converge. I got the following error messages:

Warning: the specified model did not converge
Error: error in computing the inverse link function

It's odd because if I use fractional powers....the model converge: too bad that fractional powers don't do in my case.

Double odd is the fact that I ran a similar one for another QOL index (range, 0.34 to 1.00), same distributional issues, same power link but with no convergence issues.

Thanks for advice,

L.
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