Re: [R] Problem plotting curve on survival curve (something silly?)

From: Calum <stats_at_wittongilbert.free-online.co.uk>
Date: Mon, 03 Mar 2008 09:16:05 +0000

> Calum wrote:
>
>> All of that is very nice so far. The I followed bits and pieces of
>> other peoples posts in the past to plot on a weibull regression...
>>
>> > my_curve.Plac <- survreg( Surv(Survival, Censored==0)~
>> TreatmentGroup, subset=TreatmentGroup=="Placebo", data=TestData,
>> dist='weibull')

Peter wrote:
> I'd take a hard look at the pweibull(...) bit. Is "scale" really what
> you want it to be? If coef(my_curve.Pred) is not a scalar, then it gets
> recycled, which could easily cause oscillations.

Aha - found it. Its from my attempt to subgroup the data before I discovered subset... I now have a curve instead of an oscillation. Now possibly that curve is wrong... will need to do more reading!

For reference the line above should read:

   > my_curve.Plac <- survreg( Surv(Survival, Censored==0)~   1, subset=TreatmentGroup=="Placebo", data=TestData,   dist='weibull')

>> Also is it possible to get an R-squared type value for the fit of this
>> curve from someplace?
>>
>> Finally (three questions in one!) the first two censored data points
>> (1 in each group) are actually lost to follow-ups. Should they be
>> marked differently from censored?
>>
> Customarily they are not. (I'm sure it is possible to speculate at
> length about it, though.)
>
Going off topic a bit - but did you mean customarily they are not censored or customarily they are not handled differently from censored!

> Nothing spectacularly incompetent this far... (I'm not happy with R^2
> measures outside of linear models, or even within linear models, but
> several well-reputed people do find them useful, so who am I to bicker?)

I'm not competent to argue. But are you suggesting there is a better way to assess fit of the line to the data? Thats what I want - Not being a statistician I'm not fussed how its done. But If I'm going to extrapolate a line I'd like to know its a reasonable fit first (is that purely by eye?) There is a p value reported by survreg but no idea how to interpret it ;-)



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