multiple comparison of interaction of ANCOVA

Hi there,

The following data is obtained from a long-term experiments.

mydata <- read.table(textConnection("

+ ? ? ? ?y year Trt
+ ? ? 9.37 1993 ? A
+ ? ? 8.21 1995 ? A
+ ? ? 8.11 1999 ? A
+ ? ? 7.22 2007 ? A
+ ? ? 7.81 2010 ? A
+ ? ?10.85 1993 ? B
+ ? ?12.83 1995 ? B
+ ? ?13.21 1999 ? B
+ ? ?13.70 2007 ? B
+ ? ?15.15 2010 ? B
+ ? ? 5.69 1993 ? C
+ ? ? 5.76 1995 ? C
+ ? ? 6.39 1999 ? C
+ ? ? 5.73 2007 ? C
+ ? ? 5.55 2010 ? C"), header = TRUE)

closeAllConnections()

The experiments is designed without replication, thus I have to use ANCOVA
or linear mixed effect model to analyze the data. In the model, variable
year is coded as a continuous variable, and Trt is factor variable.

mydata.aov <- aov(y~Trt*year, mydata)
anova(mydata.aov)

Analysis of Variance Table

Response: y
? ? ? ? ?Df ?Sum Sq Mean Sq ?F value ? ?Pr(>F)
Trt ? ? ? ?2 140.106 ?70.053 197.9581 3.639e-08 ***
year ? ? ? 1 ? 0.610 ? 0.610 ? 1.7246 ?0.221600
Trt:year ? 2 ? 8.804 ? 4.402 ?12.4387 ?0.002567 **
Residuals ?9 ? 3.185 ? 0.354
---
Signif. codes: ?0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

As you have seen, the interaction effect is significant. I hope to use
TukeyHSD() or glht() to do multiple comparison on Trt:year. However, for
variable year is not a factor, they all give error messages.

I try to follow the demo("MMC.WoodEnergy") in HH package, as follwoing:

library(HH)
mca.1993 <- mcalinfct(mydata.aov, "Trt")
non.zero <- mca.1993[,5:6] != 0
mca.1993[,5:6][non.zero] <- 1993 * sign(mca.1993[,5:6][non.zero])
summary(glht(mydata.aov, linfct=mca.1993))

? ? ? ? Simultaneous Tests for General Linear Hypotheses

Fit: aov(formula = y ~ Trt * year, data = mydata)

Linear Hypotheses:
? ? ? ? ? Estimate Std. Error t value Pr(>|t|)
B - A == 0 ? 2.8779 ? ? 0.5801 ? 4.961 ?0.00215 **
C - A == 0 ?-2.8845 ? ? 0.5801 ?-4.972 ?0.00191 **
C - B == 0 ?-5.7624 ? ? 0.5801 ?-9.933 ?< 0.001 ***
---
Signif. codes: ?0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
(Adjusted p values reported -- single-step method)

It can give comparison between levels of Trt within one year, e.g., 1993.

Is it possible to do multiple comparison for the following pairs:

A.1995 - A.1993
B.1995 - A.1993
C.1995 - A.1993

Any suggestions or comments will be really appreciated. Thanks in advance!

Regards,
Jinsong