Dear colleagues,
I have an experiment where
cross-sections of cotton fibres are measured for several parameters. There are
a variable number of cross-sections per slide and a number of slides per
sample. Since the number of cross-sections per slide varies greatly the design
is unbalanced. The slides are treated as random effects. For subsequent
measurements we want to be able to establish the most efficient way to obtain
an estimate of the mean with a particular variance. That is, we want to
establish the number of cross-sections per slide and the number of slides
necessary to get a particular sample variance. My standard stats tell me that s(sup2)(suby)=s(sup2)(sub1)/n1
+ s(sup2)(sub2)/(n1*n2) but I am not sure how to get the between and within
estimates of variance. Varcomp will provide values if I set is.random=TRUE for
the data frame but these results, apart from occasionally being negative, do
not seem to agree with the MSE values generated by ssType3 for the between
component. S+ v6.0 release 2 does not seem to be able to handle unbalanced
designs with random effects - or so it says. Can S+ help me or am I in
way too deep?
Thanks,
Graham Higgerson