On May 21, 2010, at 11:48 AM, Thomas Lumley wrote:
On Thu, 20 May 2010, David Winsemius wrote:
Almost. Testing with the examples on svytable help page:
test <- function(Z){
fmla <- as.formula( paste(" ~ ", paste(c(Z, "stype"), collapse=
"+")))
chisq <- svychisq( fmla, design=rclus1,
statistic="adjWald", round=4)
}
reslt <- test("sch.wide")
reslt
Design-based Wald test of association
data: svychisq(fmla, design = rclus1, statistic = "adjWald", round
= 4)
F = 2.2296, ndf = 2, ddf = 13, p-value = 0.1471
The "con" object probably does not exist, so one needs to pass a
character name to the formula.
That works, but you can see why I prefer
test<-function(formula){
svychisq(update(formula, ~.+stype), dclus1)
}
test(~sch.wide)
I certainly can see the reason for your preference. Much cleaner. I
also looked at the help page for update.formula as suggested by the
update help page. So, when the R interpreter encounters the "~" in the
test call, it acts in a manner something akin to encountering back-
quote or "$" and defers evaluation of "sch.wide"?
Model formulas were invented for precisely this purpose. You can
work around them using character strings or using bare symbol names
and substitute, but it's really easier just to work with the formula.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
David Winsemius, MD
West Hartford, CT
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