R help - Jun 2012 - predicting expected number of events using a coxph model

I fit a coxph model:

coxphfit <- coxph(Surv(sampledLifetime, !sampledCensoredQ) ~  curpbc6  +
prevpbc6,  sampledTimeSeries)

Now I'm trying to predict the expected number of events using a new dataset.
The documentation suggests that

coxPred <- predict(coxphfit, newdata = testTimeSeries,
type="expected")

will do what I want, but I get the error

Error in model.frame.default(data = testTimeSeries, formula
Surv(sampledLifetime,  :
  variable lengths differ (found for 'curpbc6')

when I do this. The dataframes sampledTimeSeries and testTimeSeries were
constructed by taking rows from a larger dataframe, so they have the same
data.

What am I doing incorrectly?

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On Jun 29, 2012, at 23:56 , agittens wrote:
> I fit a coxph model:
> 
> coxphfit <- coxph(Surv(sampledLifetime, !sampledCensoredQ) ~  curpbc6  +
> prevpbc6,  sampledTimeSeries)
> 
> Now I'm trying to predict the expected number of events using a new
dataset.
> The documentation suggests that
> 
> coxPred <- predict(coxphfit, newdata = testTimeSeries,
type="expected")
> 
> will do what I want, but I get the error
> 
> Error in model.frame.default(data = testTimeSeries, formula >
Surv(sampledLifetime,  :
>  variable lengths differ (found for 'curpbc6')
> 
> when I do this. The dataframes sampledTimeSeries and testTimeSeries were
> constructed by taking rows from a larger dataframe, so they have the same
> data.
> 
> What am I doing incorrectly?
Most likely referring to a variable not in testTimeSeries. (I kind of suspect
that unlike predict.lm, predict.coxph does not ignore the left hand side of
formulas. Does testTimeSeries contain a sampledLifetime column?)

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com