Re: [R] Make natural splines constant outside boundary

From: Charles C. Berry <cberry_at_tajo.ucsd.edu>
Date: Sat, 8 Dec 2007 17:38:44 +0000 (UTC)

Gad Abraham <g.abraham <at> ms.unimelb.edu.au> writes:

>
> Hi,
>
> I'm using natural cubic splines from splines::ns() in survival
> regression (regressing inter-arrival times of patients to a queue on
> queue size). The queue size fluctuates between 3600 and 3900.
>
> I would like to be able to run predict.survreg() for sizes <3600 and
> >3900 by assuming that the rate for <3600 is the same as for 3600 and
> that for >4000 it's the same as for 4000 (i.e., keep the splines cubic
> within the boundaries but make them constant outside the boundaries).
>

[snip]

> Any suggestions?

Here is one.

> range(ovarian$age)
[1] 38.8932 74.5041
> trim <- function(x) pmin(74.5041 ,pmax(38.8932 , x))
> s <- survreg(Surv(futime, fustat) ~ ns(age, knots=c(50,

60),Boundary.knots=c(38.8932, 74.5041)),data=ovarian)

> s2 <- survreg(Surv(futime, fustat) ~ ns(trim(age), knots=c(50,
60),Boundary.knots=c(38.8932, 74.5041)),data=ovarian)
> matplot(newage, cbind(predict(s,newdata=newage),predict(s2,newdata=newage)))
>

HTH, Chuck

>
> Thanks,
> Gad
>



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