I want to see if there is a significant difference in BHB concentration between 5 age categories in some farm animals. Farm should be a random effect in the model. The issue is that BHB concentration is left-truncated (lower detection limit at 96). I have 94 data rows, of which 63 are at the lower limit of detection.
If BHB had a normal distribution, I would just use a linear mixed model like this and do a post-hoc test like Tukey for pairwise comparisons between age categories:
lmer(BHB ~ age_categories + (1|farm), data=file)
Given the truncated distribution, I think I should rather use tobit regression (using VGAM or censReg package in R) but the issue is I cannot add factor variable (age categories) in such models (I have an error when I had a factor effect).
Any suggestions?
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