st: left-truncation of entry in survival analysis

[Sue Chinn <sue.chinn@kcl.ac.uk>]

Dear Statalist readers,

Reports of survival analysis which use age as the time scale rather than 
time-on-study often 'adjust for delayed entry'. In Stata this is achieved by:

stset age, fail(died) enter(ageatentry)

(see recent e-mail from Dawn Teele, or reply to st: streg from 
rgutierrez@stata.com on 19th September 2002.)

However, a model fitted with the above stset gives exactly the same answer 
as one with

stset timeonstudy, fail(died)

provided timeonstudy=age-ageatentry (as it normally would, but might not 
exactly depending how variables were calculated) and the models are exactly 
the same. In the second model it is usual to adjust or stratify on age, 
while in the first it isn't as age is taken into account, supposedly, so 
researchers may not have realised the equivalence.

So, am I missing something, or are advocates of the first model deluding 
themselves? Can left truncation be ignored with age as the timescale?

Thanks

Sue

Sue Chinn
Professor of Medical Statistics
Division of Asthma, Allergy and Lung Biology
King's College London
5th Floor Capital House
42 Weston Street
London SE1 3QD

tel no. 020 7848 6607
fax no. 020 7848 6605



*
*   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/