Hello there,

I am using nlme() to fit a logistic mixed effect model on our data.
The outcome variable is binary.
I got the error when I wanted to add a group factor to my model.

My initial model is as below:

model.a <- nlme(response~ 1/(1 + exp( -intercept- u0 - slope*TIME -
u1*TIME)),
                           + fixed=intercept+slope~1, random= u0+u1~1
|studentID,
                           + start=c(slope=.01, intercept=-1), data=log.data,
method='ML')

This works fine on my data. But when i update it by adding a group factor
SKILLS,
I got the error message:
"Error in nlme.formula(response ~  1/(1 + exp( -intercept- u0 - slope*TIME
:
        starting values for the fixed component are not the correct length"

The model is as below:
model.a <- nlme(response~ 1/(1 + exp( -intercept- u0 - slope*TIME -
u1*TIME)),
                           + fixed=intercept+slope ~ SKILLS, random= u0+u1~1
|studentID,
                           + start=c(slope=.01, intercept=-1), data=log.data,
method='ML')

Does anybody see anything wrong with the "start" part of this model?

Thanks a lot!!

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