From: Jim Brindle <j_brindle_at_hotmail.com>

Date: Sat 08 Oct 2005 - 22:07:33 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Oct 08 22:11:37 2005

Date: Sat 08 Oct 2005 - 22:07:33 EST

Hello,

I am trying to perform a two-factor ANOVA analysis using a blocking design with "Vol" as the response variable. My intent is to have "Rater" treated as the treatment variable and the "Pipe" treated as the blocking variable. I am reading and preparing my dataset using the following three lines of code:

values <- read.table("filename", header=TRUE)
attach(values)

values = data.frame(values)

The dataset is the following:

Pipe Rater Volume

1 A 5.129 1 B 5.296 1 C 4.679 1 D 4.776 2 A 8.519 2 B 8.482 2 C 7.659 2 D 7.798 3 A 13.769 3 B 14.621 3 C 12.418 3 D 13.189

Below there are 2 versions which I've used to run my analysis.

Option #1:

g <- lm(Volume ~ factor(Rater) + factor(Pipe), values) print(anova(g))

Option #2:

Rater <- as.factor(Rater)

Pipe <- as.factor(Pipe)

g <- lm(Volume ~ Rater + Pipe, values)

print(anova(g))

A couple of questions I have are:

- I thought that option #1 and option #2 would have given me the same results and they don't appear to. The only difference (to me) is how I have specified the factors used in the model. However, there appears to be something else I am missing and I was just wondering if anyone has any insight as to which is the correct way to code this analysis?
- Unless otherwise specified is there a particular reference level that R uses by default - for example in this case, the second treatment level (Rater B)?
- Is there a good reference someone can point me to for more insight on the two-factor ANOVA analysis with R?

Thank you kindly in advance,

Jim

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R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Oct 08 22:11:37 2005

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