Dear Help List,
Given a standard one-way ANCOVA model for comparing k groups and with one
covariate factor e.g. ANC <- lm(Y ~ X + Group) run within package "car"
followed by the estimation of adjusted group means with the "effects" package
e.g. Effects1 <- Effect("Group", ANC, se = TRUE) how can I extract the
individual case values adjusted to the covariate grand mean that theoretically
go into creating each adjusted group mean and its confidence interval? I
recognize that summary computational formulae are probably used to estimate
adjusted group means and their SE and the package "effects" may not even
calculate individual case adjusted values. Use of the "fitted" command applied
to the ANC model e.g. Fitted1 <- fitted(ANC) produces case values that do not
average to the adjusted mean for each group output by "Effects" while applying
"fitted" to the Effects output results in NULL. I am unsure why the first
result occurs and the second outcome may result from having applied a command i!
nappropriately. A function may need to be created to estimate case adjusted
values but that is, at this time, beyond me. I have read the "effects" package
pdf but found nothing to enlighten me on this issue. I have estimated
adjusted case values in Excel using Yij(adj) = yij - b(xij - xbar) where b is
the slope of the pooled Group regression and xbar is the grand mean X but the
mean of the adjusted case values for a group do not always match the adjusted
group means from the Effects or even the fitted command output as closely as I
think they should (to at least 3 decimals given the data are log transformed)
even after allowing for decimal rounding because I used only 5 places in Excel.
Thanks for any assistance in how to extract the case values used to create the
adjusted group means from an ANCOVA by the "effects" package such that the mean
of the case values for a group equals the adjusted mean for that group.
Regards,
B. Jessop
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