I have data on the severity of weed pressure for an agricultural study with a factorial, RCBD.  The weed pressure variable is the area in square footage within a plot where weeds were measured to be above a certain height.  It is an atypical metric for weediness, and I have not been able to find anything in the literature like it, so I'm unsure of how to analyze this data.   The raw, untransformed variable "Area_Weed", in units of square feet, exhibits right-skewness.  A few of the observations are zero.  The histogram is shown below:   One approach that I've tried is to convert the response variable "Area_Weed", in square feet, to a proportion of weed area divided by total plot area, the apply an arcsine transformation.  The residuals plot exhibits non-constant variance.  The code and residuals are shown below: proc glimmix data=df plots=studentpanel method=rspl; class Trt_Amend_App Trt_CC Block; model Area_Weed_PROP_ANG = Trt_Amend_App | Trt_CC / ddfm=kr2; random Block; run;   Considering an alternative distribution for the untransformed response variable "Area_Weed", which is positive and right-skewed, the gamma distribution seems promising, aside from the fact that it can't accommodate zeros in the data.  If I apply a transformation of adding a small constant (+1) then I can get the model to run using PROC GENMOD, with the caveat that I can't include the variable "Block" as a random effect. proc genmod data=df plots=all; class Trt_Amend_App Trt_CC Block; model Area_Weed_PLUS = Trt_Amend_App | Trt_CC / dist=gamma link=log; ods output ParameterEstimates=pe; output out=outmean pred=mu; run; My questions amount to the following: Based on what I've presented here, is the gamma distribution appropriate for my response variable, which is positive, right-skewed, and includes a few zeros? If I use PROC GENMOD and a gamma distribution to model, how might I go about evaluating the residual plots?  What might be a good option for the parameter plots aside from "all"? If I attempt to incorporate Block as a random effect using PROC GLIMMIX, the model does not converge.  How can I address this issue? Thank you for reading.  Please let me know if I can provide any other information. ... View more