[R] What is the effect of how.many in the "contrast" function

[jimj]

Thanks Dr. I did read the help page but maybe I just don't understand enough
about contrasts to take what I need from it. It says:

"If value supplies more than how.many contrasts, the first how.many are
used. If too few are supplied, a suitable contrast matrix is created by
extending value after ensuring its columns are contrasts (orthogonal to the
constant term) and not collinear."

I suppose what this means is that if the matrix is not extended then there
is some type of problem with orthogonality? Perhaps my real question should
be, why is a contrast matrix with only one column not suitable. 

Maybe I should read the book you are referring to. Can you provide a
reference? Thanks!






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