[R-sig-ME] toxicology dietary subchronic feed/weight analysis

[Gosse, Michelle]

Hi all,

Still continuing on the dataset I have for the effect of two toxins on rats, I managed to fit very nice linear mixed effects models for the effects on the toxins on body weight over time, using weight = toxin * time + (time + 1 | subject.ID) as per Steven Pierce's suggestion.

I didn't need to get into a nonlinear mixed effect model as the results were very nice staying within a linear framework.

The final analysis I need to do is to examine feed intake and see how this is associated with body weight, time, and drug. I have feed intake measured daily, so 28 intake data points per subject, but only 7 weight data points, so while I have repeated measures for both, I do not have a fully linked intake-weight series.

The research question is whether the toxin influenced feed intake (feed palatability issue). I'm interested in intake slopes/partial slopes, but obviously body weight should be the main driver of feed intake (heavier rats eat more).

I'm thinking of an analysis similar to: intake = toxin*body weight*time (time +1|subject.ID)

But I'm not sure I have the sample size to do a three-way effect, and I don't know that this is the correct model specification given that I have weight data which is not missing at random - all the rats were measured on  specific days such as Day 1, Day 4, Day 7.

Has anyone worked with a similar dataset to advise what model to fit.

Cheers
Michelle, note: I do not work Fridays


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