Re: st: left-truncation of entry in survival analysis

[Ron�n Conroy <rconroy@rcsi.ie>]

On 22 M�rta 2006, at 12:46, Sue Chinn wrote:

> So, am I missing something, or are advocates of the first model  
> deluding themselves? Can left truncation be ignored with age as the  
> timescale?
One powerful argument for using age as the time variable in survival  
analysis has to do with the hazard function itself. In modelling risk  
factors or prognostic factors, the shape of the hazard function is  
often treated as a high-dimensional nuisance parameter (I love that  
expression and use it as often as I can). However, in risk estimation  
the hazard function represents the absolute risk, without which the  
hazard ratios cannot be interpreted in real-life terms.

Using age as the time variable allows the construction of hazard  
functions spanning the age range from the youngest entry to the  
oldest exit. It allowed us, in the SCORE project, to make  
cardiovascular risk charts showing ten-year risk from datasets many  
of which contained at most 8 years of follow up.

Since the publication of Ed Korn's paper, there have been a number of  
papers looking to see if the modelling of risk factor effects is  
affected by the choice of time metric and the short answer seems to  
be 'not so you would notice'. I haven't been keeping up with the  
literature on this, but I don't know of a case where someone got very  
different results - does anyone else?

Korn's proposal appeals to my sense of elegance, as well as being a  
gift to those of us who have to deal with absolute risks.

Ron�n Conroy
rconroy@rcsi.ie




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