Message-ID: <47CBC1D5.9020004@wittongilbert.free-online.co.uk>
Date: 2008-03-03T09:16:05Z
From: Calum
Subject: Problem plotting curve on survival curve (something silly?)
In-Reply-To: <47CB2F15.1070606@biostat.ku.dk>
> 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 ;-)