[Rd] drop.unused.levels

From: Terry Therneau <therneau_at_mayo.edu>
Date: Thu, 31 Mar 2011 09:06:04 -0500


 A user sent me a query on survreg, which when boiled down is a request to drop unused levels when setting up the X matrix.  I don't have a strong opinion one way or the other, but am loath to make the change: I expect that code somewhere will break, perhaps a lot, when the length of the coefficients changes.

 On the other hand, at some point this change was made to lm(). Does the R core have an opinion on which is better, or a pointer to discussion that underlied the change to lm()? It could be enough to push me over the edge. It would affect survreg, coxph, survfit, survdiff, coxme, ...

Terry Therneau



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