[R] Question on mixed effect models with LME
peterflomconsulting at mindspring.com
Wed Oct 21 19:06:58 CEST 2009
Using R 2.9.2 on a machine running Windows XP
I have a longitudinal data set, with data on schools and their test scores over a four year period. I have centered year, and run the following
m1.mod1 <- lme(fixed = math_1 ~ I(year-2007.5)*TFC_,
data = long,
random = ~I(year-2007.5)|schoolnum,
na.action = "na.omit")
where math_1 is a percentage of students in a given school that are at the lowest math achievement level, year is year, TFC_ is a categorical variable for a treatment I wish to evaluate, and schoolnum is an identifier.
When I run summary on this model, I get a strong negative correlation (-0.91) between the intercept and I(year-2007.5), despite the fact that the mean of year is 2007.5.
I am puzzled, as I thought centering the time variable should eliminate, or at least strongly reduce, this correlation.
Any insights appreciated
Peter L. Flom, PhD
Website: www DOT peterflomconsulting DOT com
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