[R-sig-ME] fixing the value of some parameters in an lme or lmer model

[Asher Strauss]

Dear list,

I am trying to fit a two level hierarchical model using the lme/lmer
function.
I have two levels in my data: 3 time points (codded: week 0, week 4, week
8) nested within subjects (ID). I would like to set the intercept of each
patient to the value of the outcome at week 0 (the first observation), or
in other words force the model fit for each subject to pass through the
subject's baseline score on the outcome variable.

Here are the formulas (in informal notation):

Level 1: (time)
outcome=b0+b1*week+error

Level 2: (subjects)
b0=*0*+*1**"outcome-at-time-0"
b1="grand-slope"+error

I underlined and made bold the values I would like to force.

Can this be done in nlme or lme4 packages?

Thank you very much!!!

Asher Strauss

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