topic Re: Glimmix procedure using a weibull distribution in Statistical Procedures

Glimmix procedure using a weibull distribution

boriquez — Mon, 12 Dec 2016 21:02:47 GMT

I would like to analyze a multi-location complete randomized block design where resposne variable is days to maturity (of a crop). After running a proc severity the weibull distribution was selected. My questions are :

Is there any information about using the weibull distribution in the glimmix procedure, if so which is the appropriate link function?

Do I need a fully saturated model with weibull distribution as in a binomial distribution?

Re: Glimmix procedure using a weibull distribution

Rick_SAS — Mon, 12 Dec 2016 21:09:27 GMT

You say "after running PROC SEVERITY, a Weibull distribution was selected." Did you use PROC SEVERITY to run a fixed-model regression, or did you just analyze the response variable in isolation (a univariate model)?

Re: Glimmix procedure using a weibull distribution

boriquez — Mon, 12 Dec 2016 21:12:06 GMT

In Isolation (Univariate model)

Re: Glimmix procedure using a weibull distribution

boriquez — Mon, 12 Dec 2016 21:16:10 GMT

Thank you for taking the time to help Rick.

Re: Glimmix procedure using a weibull distribution

Rick_SAS — Mon, 12 Dec 2016 21:33:16 GMT

I don't think a univariate histogram/analysis of the response variable tells you very much. The DIST= option in GENMOD and GLIMMIX specify the (conditional) error distribution, which might be very different from the distribution of the response. See the pictures in the article "Error distributions and exponential regression models."

To answer your implied question, the GLIMMIX procedure does not support a Weibull distribution function for the errors. It sounds like you are treating the response variable as a continuous variable. If you think you have long-tailed errors, you might look at the gamma or lognormal distributions. (The gamma is sometimes used for waiting times.) Look at the goodness-of-fit statistics and the diagnostic residual plots to guide you in choosing a good model.

Re: Glimmix procedure using a weibull distribution

boriquez — Mon, 12 Dec 2016 21:52:17 GMT

Once again thank you Rick, for pointing me in the right direction. I will take a look at your sugestions and explore both the lognormal and gamma distribution.