[R] less than full rank contrast methods

[Frank Harrell]

Given a non-singular fit, the contrast function in the rms package will allow
you to request multi-dimensional contrasts some of which are redundant. 
These singular contrasts are automatically ignored.  One use for this is to
test for differences in longitudinal trends between two of three treatment
groups, where the time trend is a multiple degree of freedom
parameterization such as cubic splines.  You don't have to stop and think
about how many time points to test; just test as many as you'd like and get
the right degrees of freedom according to the number of spline terms (main
effects + interactions).

Frank

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Frank Harrell
Department of Biostatistics, Vanderbilt University
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