[R] treatment effect at specific time point within mixed effects model

Afshartous, David afshart at exchange.sba.miami.edu
Tue Sep 26 17:13:01 CEST 2006


The code below is for a pseudo dataset of repeated measures on patients
where there is also a treatment factor called "drug".  Time is treated
as categorical.  

What code is necessary to test for a treatment effect at a single time
e.g., time = 3?   Does the answer matter if the design is a crossover
i.e, each patient received drug and placebo?

Finally, what would be a good response to someone that suggests to do a
simple t-test (paired in crossover case) instead of the test above
within a mixed model?


z = rnorm(24, mean=0, sd=1)
time = rep(1:6, 4)
Patient = rep(1:4, each = 6)
drug = factor(rep(c("I", "P"), each = 6, times = 2)) ## P = placebo, I =
dat.new = data.frame(time, drug, z, Patient)
data.grp = groupedData(z ~ time | Patient, data = dat.new)
fm1 = lme(z ~ factor(time) + drug + factor(time):drug, data = data.grp,
random = list(Patient = ~ 1) )

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