[R] Error using newdata argument in survfit

[Hanke, Alex]

Hi Brian,
The factor Prior.f has 5 levels (1,2,3,4,5) which coxph deals with by
creating 4 dummy variables coded with 1 or zero. That's what I see when I
look at fit$x.
fit$x[1:5,]
   Week LagAOO factor(Prior.f)2 factor(Prior.f)3 factor(Prior.f)4
31   22      0                0                0                0
32   22      0                0                0                0
33   22      2                0                0                0
34   22      3                0                0                0
35   22      2                0                0                0
   factor(Prior.f)5
31                0
32                0
33                0
34                0
35                0 
I have played with the formula a bit adding a term at a time and then
checking to see if I can produce the survival curves for pseudo cohorts. I
get as far as the Prior.f term and am successful if I treat it as a
continuous variable. If I introduce it as a factor and assume it wants four
dummy variables as above I get the variable lengths error. If I represent
the term with one variable:
survfit(fit,list(Week=c(15,15),LagAOO=c(0,0),Prior.f=c(1,2)))
I get:
Error in x2 %*% coef : non-conformable arguments
Which is a nice change but still short of knowing what is going on.
 Regards
Alex 

-----Original Message-----
From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
Sent: June 15, 2005 4:24 PM
To: Hanke, Alex
Cc: 'r-help at stat.math.ethz.ch'
Subject: Re: [R] Error using newdata argument in survfit

You appear to have a coding for prior.f in newdata rather than the factor 
itself.

It's a bit hard to be sure when we don't have data8 to compare with.

On Wed, 15 Jun 2005, Hanke, Alex wrote:

> Dear R-helpers,
> To get curves for a pseudo cohort other than the one centered at the mean
of
> the covariates, I have been trying to use the newdata argument to survfit
> with no success. Here is my model statement, the newdata and the ensuing
> error. What am I doing wrong?
>
>> summary(fit)
> Call:
> coxph(formula = Surv(Start, Stop, Event, type = "counting") ~
>    Week + LagAOO + Prior.f + cluster(interaction(Station, Year)),
>    data = data8, method = "breslow", x = T, y = T)
>
>  n= 1878
>            coef exp(coef) se(coef) robust se     z       p
> Week     0.00582      1.01   0.0323    0.0239 0.244 8.1e-01
> LagAOO   0.71929      2.05   0.1238    0.1215 5.918 3.3e-09
> Prior.f2 0.12927      1.14   0.4402    0.4025 0.321 7.5e-01
> Prior.f3 0.79082      2.21   0.5484    0.4460 1.773 7.6e-02
> Prior.f4 2.04189      7.71   0.6008    0.4685 4.358 1.3e-05
> Prior.f5 1.20450      3.34   0.6423    0.5481 2.198 2.8e-02
>
>         exp(coef) exp(-coef) lower .95 upper .95
> Week          1.01      0.994     0.960      1.05
> LagAOO        2.05      0.487     1.618      2.61
> Prior.f2      1.14      0.879     0.517      2.50
> Prior.f3      2.21      0.453     0.920      5.29
> Prior.f4      7.71      0.130     3.076     19.30
> Prior.f5      3.34      0.300     1.139      9.76
>
> Rsquare= 0.047   (max possible= 0.25 )
> Likelihood ratio test= 91  on 6 df,   p=0
> Wald test            = 209  on 6 df,   p=0
> Score (logrank) test = 142  on 6 df,   p=0,   Robust = 17.4  p=0.00803
>
>  (Note: the likelihood ratio and score tests assume independence of
>     observations within a cluster, the Wald and robust score tests do
not).
>>
> newdat
>      Week   LagAOO Prior.f2 Prior.f3 Prior.f4 Prior.f5
> 1 17.55218 1.191693        1        0        0        0
> 2 17.55218 1.191693        0        0        0        0
>
>> survfit(fit,newdata=newdat)
> Error in model.frame(formula, rownames, variables, varnames, extras,
> extranames,  :
>        variable lengths differ
> In addition: Warning message:
> 'newdata' had 2 rows but variable(s) found have 1878 rows
>
> Regards,
> Alex
>
>
> Alex Hanke
> Department of Fisheries and Oceans
> St. Andrews Biological Station
> 531 Brandy Cove Road
> St. Andrews, NB
> Canada
> E5B 2L9
>
>
>
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>
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-- 
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)
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