Wednesday, March 22, 2006

st: left-truncation of entry in survival analysis

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

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