[Rd] model frames and update()

Therneau, Terry M., Ph.D. therneau at mayo.edu
Thu Apr 23 18:58:14 CEST 2015

This issue has arisen within my anova.coxph routine, but is as easily illustrated with glm.

testdata <- data.frame(y= 1:5,
                        n= c(8,10,6,20,14),
                        sex = c(0,1,0,1,1),
                        age = c(30,20,35,25,40))

fit <- glm(cbind(y,n) ~ age + sex, binomial, data=testdata, model=TRUE)
saveit <- fit$model

update(fit, .~. - age, data=saveit)
Error in cbind(y, n) : object 'y' not found

One would hope that a saved model frame is precisely the thing that would work best. The 
issue of course is that "cbind(y, n)" is the name of the first variable in saveit, and it 
is not being properly quoted somewhere down the line.  The same issue can occur on the 
right hand side.  "Save the model frame in case you need to refit something next month" is 
does not appear to be a safe approach to reproducable research.

fit2 <- glm(y ~ sex + log(age), poisson, testdata)
save2 <- fit2$model
update(fit2, . ~ . - sex, data=save2)  # fails
glm(y ~ log(age), poisson, save2)      # fails

I can work around this in my anova, but I wanted to not rebuild the frame if it is already 
present.   It looks like model.matrix plus attr(x, 'assign') time -- a bit harder to read, 
but that looks like what anova.glm is doing.  Is there a way to make update work?

The current code, BTW, starts by building its own frame using results of terms.inner, 
which solves the above issue nicely and update() works as expected.  But it isn't robust 
to scoping issues.  (As pointed out yesterday by a user: lapply of a function that 
contained coxph followed by anova gives a variable not found error.)  It also ignores 
saved model frames; thus the rewrite.

Terry T

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