[R] Question on mixed effect models with LME
Peter Flom
peterflomconsulting at mindspring.com
Wed Oct 21 19:06:58 CEST 2009
Good afternoon
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
thanks
Peter
Peter L. Flom, PhD
Statistical Consultant
Website: www DOT peterflomconsulting DOT com
Writing; http://www.associatedcontent.com/user/582880/peter_flom.html
Twitter: @peterflom
More information about the R-help
mailing list