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

S Ellison S.Ellison at lgc.co.uk
Thu Mar 4 15:03:14 CET 2010

>>> "John Fox" <jfox at mcmaster.ca> 02/03/2010 02:19 >>>
>There's also a serious question about whether one would
>be interested in main effects defined as averages over the level of
>other factor when interactions are present. 

My personal take on this particular chestnut is that I often want to
ask something about the relative size of the effects. If the so-called
"main effect(s)" is/are very much larger than the interactions, one may
be able to make generalisations which have practical use. If the effects
are much of a size, there's nothing much to be gained by asking about
"main effects".  

Mind you, that's probably a crit of significance testing as the be-all
and end-all, rather than a problem with type I-III. Asking 'how big is
it?' is a step beyond 'is it there?'.

Steve E

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