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Level II

Analysis of covariance with covariate at higher level

I am analysing an experiment that involved growing plants at different densities.  I used several different plant genotypes that each experienced a low and high density treatment in a completely randomised design.  In addition to measuring the fruit yield of plants (response variable), I also have a measure of plant size for each genotype, measured in a different environment (apart from this experiment). I want to use this plant size measure as a covariate in the analysis of the density experiment, and I'm particulalry interested in testing for an interaction between the covariate and the density treatment.  


The data are set up like this mock example:


genotypedensity treatmentcovariateresponse (yield)










I am using the Fit Model platform, and I have so far fit a model with the effects 'density treatment', 'covariate', and their interaction. But I think I need something else to account for the covariate being a measure of the genotype as a whole -- i.e., the covariate is an among-genoytpe effect and the treatment is a within-genotype effect.  


Could someone please help by identifying the proper model to be fit?

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