Message-ID: <8975119BCD0AC5419D61A9CF1A923E950121BB2B@iahce2knas1.iah.bbsrc.reserved>
Date: 2005-04-20T13:40:03Z
From: michael watson (IAH-C)
Subject: Anova - adjusted or sequential sums of squares?
Hi
I am performing an analysis of variance with two factors, each with two
levels. I have differing numbers of observations in each of the four
combinations, but all four combinations *are* present (2 of the factor
combinations have 3 observations, 1 has 4 and 1 has 5)
I have used both anova(aov(...)) and anova(lm(...)) in R and it gave the
same result - as expected. I then plugged this into minitab, performed
what minitab called a General Linear Model (I have to use this in
minitab as I have an unbalanced data set) and got a different result.
After a little mining this is because minitab, by default, uses the type
III adjusted SS. Sure enough, if I changed minitab to use the type I
sequential SS, I get exactly the same results as aov() and lm() in R.
So which should I use? Type I adjusted SS or Type III sequential SS?
Minitab help tells me that I would "usually" want to use type III
adjusted SS, as type I sequential "sums of squares can differ when your
design is unbalanced" - which mine is. The R functions I am using are
clearly using the type I sequential SS.
Any help would be very much appreciated!
Thanks
Mick