I looked at the nocenter and it says (-1,0,1) values but it seems that any
three-level factor is included in that (represented as 1,2,3 in R) .
Also, is the baseline curve now showing the reference level and not the
fictional .428 sex? If I predict the risk for a new row, should I multiply the
coefficient shown in the output by 1 for a sex=1? It used to be (1-.428)*coef.
Thanks for clarifying.
SB
From: Therneau, Terry M., Ph.D. <thern...@mayo.edu>
Sent: Friday, 3 September, 2021 12:37
To: Bond, Stephen <stephen.b...@cibc.com>
Cc: R-help <r-help@R-project.org>
Subject: Re: coxph means not equal to means of model matrix
[EXTERNAL]
________________________________
See ?coxph, in particular the new "nocenter" option.
Basically, the "mean" component is used to center later computations. This can
be critical for continuous variables, avoiding overflow in the exp function,
but is not necessary for 0/1 covariates. The fact that the default survival
curve would be for a sex of .453, say, was off-putting to many.
Terry T.
On 9/3/21 11:01 AM, Bond, Stephen wrote:
Hi,
Please, help me understand what is happening with the means of a Cox model?
I have:
R version 4.0.2 (2020-06-22) -- "Taking Off Again"
Copyright (C) 2020 The R Foundation for Statistical Computing
Platform: x86_64-w64-mingw32/x64 (64-bit)
getOption("contrasts")
unordered ordered
"contr.treatment" "contr.poly"
According to the help coxph.object has a component holding the means of the X
(model.matrix). This does not hold any more.
```
library(survival)
test1 <- list(time=c(4,3,1,1,2,2,3),
status=c(1,1,1,0,1,1,0),
x=c(0,2,1,1,1,0,0),
sex=factor(c(0,0,0,0,1,1,1)))
m1 <- coxph(Surv(time, status) ~ x + sex, test1)
m1$means
## x sex1
## 0.7142857 0.0000000
colMeans(model.matrix(m1))
## x sex1
## 0.7142857 0.4285714
```
Will new observations be scored using the zero mean from the object?? Is this
just a reporting change where $means shows the reference level and no longer
the mean of the model matrix??
Thanks everybody
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