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From |
Antoine Terracol <terracol@univ-paris1.fr> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: R: Estimating the probability of censoring |

Date |
Thu, 25 Sep 2008 22:10:39 +0200 |

Michael,

Xhat is calculated here is the survival function at time t conditional on covariates, i.e., in your case, the probability that the subjects time-to-censoring will be greater than t if dying from cancer was not a risk. If that makes sense to you, the formula is:

S(t|X)=exp(-IBH(t)*exp(XB))

where IBH(t) is the integrated baseline hazard at time t

and this is what is calculated in cens_adj, assuming that the -basec()- option of -stcox- does indeed compute the integrated baseline hazard.

Now, since the baseline survival function is

S0(t)=exp(-IBH(t))

one can write the survival function as

S(t|X)=exp(exp(XB)*ln(S0(t)))

which can be rewritten as

S(t|X)=S0(t)^(exp(XB))

that last formula avoids the missing value for ln(S0(t)) when the baseline survival is 0

It is used to generate cens_adj2, assuming that the -basesurv()- option of -stcox- calculates the proper survival function.

As you can see, the results are slightly different, so I assumed after checking the web page that maybe -basec()- calculates something slightly different from IBH(t).

Antoine

Michael McCulloch wrote:

Thank you kindly Antoine, for pointing out:

to specify the baseline cumulative hazard in -stcox-, and

to multiply that baseline cumulative hazard by the regression coefficient.

I see that the two approaches you suggest:

gen cens_adj=exp(-cum*exp(coeff_cens))

gen cens_adj2=surv0^(exp(coeff_cens))

provide very similar results (as seen by . twoway scatter cens_adj cens_adj2).

May I ask:

how those two approaches differ, and

why the probability varies so much given that died is y/n?

Michael

Hello _all

I might be missing something, but isn't the correct way to do this more like (the part where I generate cens_adj):

sysuse cancer.dta, clear

gen id = _n // generate individual IDs

stset studytime, failure(died==0) // note that total person-time is 744

*estimate the unadjusted probability of censoring

sts gen cens = s

*estimate the adjusted probability of censoring

stcox drug age, nohr basec(cum)

predict coeff_cens, xb // predicts linear coefficients of censoring

gen p_cens = exp(coeff_cens) // adjusted probability of time-to-censoring

gen cens_adj=exp(-cum*exp(coeff_cens))

*lists the results

list id drug age died cens p_cens cens_adj in 40/48, clean noobs

id drug age died cens p_cens cens_adj

40 3 50 0 .58476475 .8601028 .6811302

41 3 55 1 .58476475 .9429854 .6563864

42 3 57 1 .51166915 .9783332 .5705886

43 3 48 0 .51166915 .8290267 .6216006

44 3 56 0 .34111277 .9604967 .4199265

45 3 60 1 .34111277 1.033855 .3930005

46 3 62 0 .22740851 1.072609 .2585016

47 3 48 0 .11370426 .8290267 .2171345

48 3 52 0 0 .8923438 .0710848

Antoine

Michael McCulloch wrote:

at a very first glance, what hits the eyes is the adjusted probability ofYes, that's one of my main questions. Since I used the Stata-supplied cancer.dta file and provided all my methods, I'm hoping that someone on Statalist might have advice on how to correct the method for unadjusted estimation of censoring probability.

being censoring sometimes above the usual upper constraint. How can it be? I

should have missed something in your assumptions.

Kind Regards,

Carlo

-----Messaggio originale-----

Da: owner-statalist@hsphsun2.harvard.edu

[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Michael McCulloch

Inviato: giovedì 25 settembre 2008 17.56

A: Statalist

Oggetto: st: Estimating the probability of censoring

Hello,

I'm seeking guidance on a series of commands I've written to estimate

the probability of being censored. Might anyone be able to offer

commentary as to whether I've done this correctly? The resulting

unadjusted probability of censoring ranges from 0-1, while the

adjusted probability goes above 1.

sysuse cancer.dta, clear

gen id = _n // generate individual IDs

stset studytime, failure(died==0) // note that total

person-time is 744

*estimate the unadjusted probability of censoring

sts gen cens = s

*estimate the adjusted probability of censoring

stcox drug age, nohr

predict coeff_cens, xb // predicts linear

coefficients of censoring

gen p_cens = exp(coeff_cens) // adjusted probability of

time-to-censoring

*lists the results

list id drug age died cens p_cens in 40/48, clean noobs

40 3 50 0 .58476475 .8601028

41 3 55 1 .58476475 .9429854

42 3 57 1 .51166915 .9783332

43 3 48 0 .51166915 .8290267

44 3 56 0 .34111277 .9604967

45 3 60 1 .34111277 1.033855

46 3 62 0 .22740851 1.072609

47 3 48 0 .11370426 .8290267

48 3 52 0 0 .8923438

--

Best wishes,

Michael McCulloch

Pine Street Foundation

124 Pine St., San Anselmo, CA 94960-2674

Tel: (415) 407-1357

Fax: (415) 485-1065

mcculloch@pinestreetfoundation.org

www.pinestreetfoundation.org

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**Follow-Ups**:**Re: st: R: Estimating the probability of censoring***From:*Antoine Terracol <terracol@univ-paris1.fr>

**Re: st: R: Estimating the probability of censoring***From:*Michael McCulloch <mm@pinest.org>

**References**:**Re: st: R: Estimating the probability of censoring***From:*Michael McCulloch <mm@pinest.org>

**Re: st: R: Estimating the probability of censoring***From:*Antoine Terracol <terracol@univ-paris1.fr>

**Re: st: R: Estimating the probability of censoring***From:*Michael McCulloch <mm@pinest.org>

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