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
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