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I've got a table of 1 row per observation.

16 questions * n subjects * 4 classrooms (2 teachers * 2 conditions)

I'd like to run some simple regressions that look something like this:

  lm(length ~ elapsed)

I'd like to do the regressions several times, split out by
questionkey, student, class.  What I've been doing is using a series
of which's to create new data sets (e.g. class1, class2, class3,
class4) and running the regressions (or whatever) against each of
those new data sets.  It seems that there should be better a way to do
this.

Use by()

models<-by(data, list(student), function(thesedata)
	lm(length~elapsed,subset=thesedata))

will return a list of lm() objects.  You can use lapply() to get
information out of these, for example

lapply(models,coef)

returns a list of coefficient vectors or

sapply(models,coef)

binds the vectors into a matrix.  There's an example in help(by)

	-thomas

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