[R] Make natural splines constant outside boundary

[Gad Abraham]

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).

(By default, natural splines will be linear outside the boundaries, 
which is very bad here because the predictions grow without bound. 
Setting the boundary knots to extreme values such as (0, 10000) doesn't 
help, because there are no intermediate knots to constrain the spline 
and you get a very large cubic curve between these boundaries and the 
observed sizes.)

Any suggestions?

Thanks,
Gad

-- 
Gad Abraham
Department of Mathematics and Statistics
The University of Melbourne
Parkville 3010, Victoria, Australia
email: g.abraham at ms.unimelb.edu.au
web: http://www.ms.unimelb.edu.au/~gabraham