multcomp Tukey-Kramer

Question

I have an experiment that is unbalanced where at three sites (L, M, H) we measure a parameter (met) in four different vegetation types (a, b, c, d). All vegetation types are present at all three sites. Vegetation types are replicated 4 times at L and M and 8 times at H.

Therefore a simple anova and TukeyHSD will not work. Packages Agricolae (HSD.test) and DTK (DTK.test) are only working for one way designs, and then there is multcomp... Does the Tukey test in the mcp function calculate Tukey-Kramer contrasts, or does it give the regular Tukey contrasts? I presume the first to be the case because the package is geared towards testing multiple comparisons for unbalanced designs, but I am unsure because p-values produced with both approaches are virtually the same. What test would then be appropriate?

Also, are there more suitable approaches towards doing such a two way anova for unbalanced data sets?

library(multcomp)

(met     <-  c(rnorm(16,6,2),rnorm(16,5,2),rnorm(32,4,2)))
(site    <-  c(rep("L", 16), rep("M", 16), rep("H", 32)))
(vtype   <-  c(rep(letters[1:4], 16), rep(letters[1:4], 16), rep(letters[1:4], 32)))

dat  <-  data.frame(site, vtype, met)

# using aov and TukeyHSD
aov.000  <-  aov(met ~ site * vtype, data=dat)
summary(aov.000) 
TukeyHSD(aov.000) 

# using Anova, and multcomp
lm.000     <-  lm(met ~ site * vtype, data=dat)
summary(lm.000)
library(car)
Anova.000  <-  Anova(lm.000, data=dat)

dat$int  <-  with(dat, interaction(site, vtype, sep = "x"))
lm.000   <-  lm(met ~ int, data = dat)
summary(lm.000)
summary(glht.000 <- glht(lm.000, linfct = mcp(int = "Tukey")))

Slow

Answer 1

For unbalanced data, anova with type III SS may be used instead of type I SS [1]. Calculation of type III anova in R [2]:

model <- (met ~ site * vtype)
defopt <- options()
options(contrasts=c("contr.sum", "contr.poly"))
print(drop1(aov(model),~.,test="F"))
options <- defopt

For unbalanced data, pairwise comparisons of adjusted means may be used. Calculation in R [4]:

library(lsmeans)
print(lsmeans(model, list(pairwise ~ site)), adjust = c("tukey"))
print(lsmeans(model, list(pairwise ~ vtype)), adjust = c("tukey"))
print(lsmeans(model, list(pairwise ~ site | vtype)), adjust = c("tukey"))
print(lsmeans(model, list(pairwise ~ vtype | site)), adjust = c("tukey"))

Lines 2 and 3 compare levels of main effects "site" and "vytpe". Lines 4 and 5 compare levels of one factor at each level of another factor separately.

I hope this helps.

References

[1] Miliken and Johnsen. 2009. Analysis of messy data. Volume 1.

[2] http://www.statmethods.net/stats/anova.html

[3] http://cran.r-project.org/web/packages/lsmeans/vignettes/using-lsmeans.pdf

Kalimantan