[R] Type-I v/s Type-III Sum-Of-Squares in ANOVA

[Ravi Kulkarni]

Hello,
  I believe the aov() function in R uses a "Type-I sum-of-squares" by
default as against "Type-III".
  This is relevant for me because I am trying to understand ANOVA in R using
my knowledge of ANOVA in SPSS. I can only reproduce the results of an ANOVA
done using R through SPSS if I specify that SPSS uses a Type-I
sum-of-squares. (And yes, I know that when the sample sizes of all groups
are equal, Type-I and Type-III produce the same answers.)
  My questions: 1) exactly what is the difference between the two types of
sums-of-squares?
                      2) how can I make R use a Type-III s-o-s? Should I? R
must have some reason for using Type-I as default rather than Type-III.
(Given a choice, believe R!)

A reference (stats book, URL...) would be helpful...

Ravi



  
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