[R] To update() or not to update()?

[Prof Brian Ripley]

On Mon, 19 May 2003, Ko-Kang Kevin Wang wrote:

> Suppose I have:
>   # Fit a base model
>   d1.ph <- coxph(Surv(start, stop, event)~
>                ejec + diavol + score + smoking +
>                beta + surg.done,
>                data = data.frame(foo))
> 
>   summary(update(d1.ph, . ~ . + td1))
>   summary(update(d1.ph, . ~ . + td2)) 
> 
> As I have many columns in my data frame, foo, called td's.  e.g. td1, td2, 
> td3, ....  And I'd like to add one column each time.  What is the 
> recommended way to do this?  Whether I should do what I did above, or 
> should I do something like:
>   td1.ph <- coxph(Surv(start, stop, event)~
>                 ejec + diavol + score + smoking + 
>                 beta + surg.done + td1,
>                 data = data.frame(foo)) 
> 
>   td2.ph <- coxph(Surv(start, stop, event)~
>                 ejec + diavol + score + smoking + 
>                 beta + surg.done + td2,
>                 data = data.frame(foo))  
> 
> I've done a system.time() on it and update() doesn't seem to be much (if 
> at all) faster.  Is there an advantage of using update() then (other than 
> that the codes look neater)?

You are calling update.default on a coxph fit, so it is just neater.  It
need not be for other model-fitting classes.

BTW, addterm (MASS) would automate this for you, adding all the columns 
one at a time and collating the results.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595