How to fix the value of random intercepts (lmer/MCMCglmm)

Hi All

I am fitting a basic linear regression, where I want to estimate a single population intercept and slope.  In addition I am fitting random intercepts and slopes such that:

lmer (Y ~Intercept + Continuous Variable + (Continuous Variable |Indiviudal Group))

However the exact value of the individual group intercepts is known from the data set.  The reasons for this are a little involved but essentially Y is a cumulative total and so at the intercept I want to fit the actual cumulative total at this point for each individual.  It is important as the slope per individual needs to be constrained to pass through the actual intercept per individual.

So I want to fit this model, estimating the population intercept and slope.  I then want to fix the individual group deviation from the population intercept (random intercepts), and from this model extract estimates of individual group random slopes.

I have been unable to find any examples of fixing intercepts, unless they are fixed as a constant.  Is it possible to code the model in such a way? The model can be run in MCMCglmm or lmer  which ever package would allow me to constrain the intercepts.

Thanks

Sam?


Dr Samantha Patrick
Research Fellow
Biosciences QU116
Francis Close Hall Campus
University of Gloucestershire
Cheltenham, GL50 4AZ, UK

Research Associate: OxNav, University of Oxford

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