Statalist The Stata Listserver


[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: Left truncation in survival: how to use stset origin and enter


From   Steven Samuels <ssamuels@albany.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Left truncation in survival: how to use stset origin and enter
Date   Sun, 13 May 2007 15:27:09 -0400

Sacrificial Line-ignore.

On May 11, 2007, at 10:59 AM, Svend Juul wrote:
..
I don't think this should be considered a case of delayed
entry: The prognostic value of the lab examination applies
to the time after the examination, and the date of diagnosis
is hardly relevant in that context. So you could:

   generate risktime = enddate - examdate
   stset risktime , failure(...)
I disagree with Svend. In the comparative trials that I have studied, the date of an examination would rarely, by itself, be a natural zero for the start-time. The exceptions that I can think of are: 1) The exam is triggered by an objective external event, such as the first occurrence of certain symptoms; in essence there is a new disease stage. 2) The exam determines eligibility for a new treatment or clinical trial.

Otherwise, the exam is unrelated to the course of the disease and so the exam date is not a natural start time. Consider--what if there had been more than one examination? Which should be chosen?

The exam provides information on time-dependent covariates, specifically, "internal time-dependent covariates" (Kalbfleisch and Prentice, 1978, The Statistical Analysis of Failure Time Data). These are often correlates of disease progression, for example CD4 counts in patients with HIV. They can have great prognostic value, which is what Diego seeks to ascertain.

This seems to leave date of diagnosis as the origin. However, without more information, I cannot tell what -stset- commands are needed. I need to know: Are the study patients part of a larger group, some of whom died prior to the exam or who did not receive the exam for other reasons? Does Diego know the dates of diagnosis for the patients without exam?

I would also want to know more about the disease and how it was diagnosed. Was it discovered: by screening asymptomatic patients? 2) during investigation of an unrelated illness? 3) because of symptoms directly or indirectly caused by the disease?

Disease diagnosed in asymptomatic patients is obviously at a very different stage than that diagnosed because of symptoms.

Steven



Diego wrote:

I have a question regarding the use of origin and enter
option when stsetting data for a survival analysis with
left truncation (delayed entry).

I have the diagnosis date and the exam date for all my
patients. The exam date can be also few years after the
diagnosis date.

Obviously the diagnosis date is the date patients start
to be at risk (end-point: death) but the aim of the
study is to look at the prognostic value of the lab
examination we do on our study population, and the follow-
up in that case is only 3 years (where the follow-up
from the diagnosis time is 7-8 years).

How should I stset my data to take into account the
diagnosis date but to look at the prognostic value of
the exam date ?

------------------------------------------------------------

I don't think this should be considered a case of delayed
entry: The prognostic value of the lab examination applies
to the time after the examination, and the date of diagnosis
is hardly relevant in that context. So you could:

   generate risktime = enddate - examdate
   stset risktime , failure(...)

Hope this helps
Svend

________________________________________________________

Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000 Aarhus C,  Denmark
Phone, work:  +45 8942 6090
Phone, home:  +45 8693 7796
Fax:          +45 8613 1580
E-mail:       sj@soci.au.dk
_________________________________________________________

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2014 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index