Dear All,

This may be a very naive question. I am trying to fit the zero-truncated Negative Binomial model to data on the number of leaves on plants (this cannot be zero, but the mean of the data is 5, so I think the model above may be appropriate).  I am trying to fit a treatment effect, a regression and an interaction. I am using the package VGAM and the family posnegbinomial in the vglm model. I have taken this advice from the book

I do not understand the residual degrees of freedom as reported by the model. These are exactly (2xN)-3. Is this correct? And if so, why? When checking the p-values for the t-values, should I use the resid df as quoted by the model, or the df from a normal Poisson?

Model output below, any help much appreciated

Rebecca Ross
Department of Plant Sciences
University of Oxford



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