# [R] Plotting Coxph model with an interaction.

Gerrit Eichner gerr|t@e|chner @end|ng |rom m@th@un|-g|e@@en@de
Fri May 21 07:55:03 CEST 2021

Hi, John,

it should work the same way as without interaction (but make sure
to use the fitted object "coxfit", not just "fit" in our call of
survfit, and note that age*rx already expands to age + rx + age:rx
so that age + rx is redundant in your formula):

coxfit <- coxph(Surv(futime, fustat) ~ age+rx+age*rx, data = ovarian)
plot(survfit(coxfit, newdata=data.frame(age=60,rx=2)))

Or do I missunderstand your question?

Hth  --  Gerrit

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Am 20.05.2021 um 21:15 schrieb Bert Gunter:
> Perhaps this might be useful:
>
> https://rpubs.com/tf_peterson/interactionplotDemo
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
>
> On Thu, May 20, 2021 at 10:29 AM Sorkin, John <jsorkin using som.umaryland.edu>
> wrote:
>
>> Colleagues,
>>
>> I hope someone can tell me how to plot a cox model that contains an
>> interaction term.
>>
>> I know that plot(survfit(. . . . )) can be used to plot a Cox model, i.e..
>> coxfit <- coxph(Surv(futime, fustat) ~ age+rx, data = ovarian)
>> plot(survfit(fit, newdata=data.frame(age=60,rx=2)))
>>
>> but I don't know how to plot a cox model with an interaction, i.e.
>>
>> coxfit <- coxph(Surv(futime, fustat) ~ age+rx+age*rx, data = ovarian)
>> plot(survfit(fit, newdata=data.frame(???????)))
>>
>> Thank you,
>> John
>>
>> John David Sorkin M.D., Ph.D.
>> Professor of Medicine
>> Chief, Biostatistics and Informatics
>> University of Maryland School of Medicine Division of Gerontology and
>> Geriatric Medicine
>> Baltimore VA Medical Center
>> 10 North Greene Street
>> GRECC (BT/18/GR)
>> Baltimore, MD 21201-1524
>> (Phone) 410-605-7119
>> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
>>
>>
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>>
>
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