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

[Steve Denham]

To get around the "missingness", consider expressing feed intake as cumulative feed intake up to each body weight.  This will match everything up, AND put BW and FI on the same unit basis (total grams).  I would use the model expressed, possibly fitting an autoregressive covariance structure. Steve Denham
Director, Biostatistics
MPI Research, Inc.
 
      From: "Gosse, Michelle" <Michelle.Gosse at foodstandards.gov.au>
 To: "r-sig-mixed-models at r-project.org" <R-sig-mixed-models at r-project.org> 
 Sent: Sunday, May 10, 2015 5:32 PM
 Subject: [R-sig-ME] toxicology dietary subchronic feed/weight analysis
   
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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