[R] bias sampling

David Winsemius dwinsemius at comcast.net
Sun Mar 18 22:27:49 CET 2012

On Mar 18, 2012, at 3:54 PM, Thomas Lumley wrote:

> On Mon, Mar 19, 2012 at 6:34 AM, David Winsemius <dwinsemius at comcast.net 
> > wrote:
>> On Mar 16, 2012, at 1:09 PM, niloo javan wrote:
>>> hi
>>> i want to analyze Right Censore-Length bias data under cox model  
>>> with
>>> covariate.
>>> what is the package ?
>> I initially left this question alone because I thought there might be
>> viewers for whom it all made perfect sense. After two days that  
>> probability
>> seems to be declining. The problem I had was the meaning of "length  
>> bias
>> data". Are you talking about a non-proportional effect in which the
>> assumption of a constant hazard ratio over time is false and other  
>> methods
>> are needed. If that is correct, then you should get a copy of  
>> Therneau and
>> Grambsch's "Modeling Survival Data" and study the chapter on  
>> "Functional
>> Form'. The package would be "survival".
> Length-biased sampling is what you get when you take a cross-sectional
> sample of an ongoing process -- long intervals are over-represented.

Thank you Thomas;

  For example people who have survived to age 75 might be  
systematically different with respect to both the distribution of  
cardiovascular risk factors and their impact on the event of interest  
(AMI. CV death, or all-cause mortality) than persons at age 45. And  
that would also not take into account the fact those risk factors  
might have changed over the interval from age 45 to age 75 in the  
> If the arrival time is known for everyone in the sample, the usual Cox
> model facilities for left truncation apply.  If the arrival times are
> not known it would be much more difficult, and would probably need
> parametric modelling.

  Am I correct in thinking that additional assumptions about the  
"length-bias" would need to be explicitly stated or modeled under a  
set of plausible scenarios before progress in any framework could be  
anticipated? It would seem that there could be many forms of such a  


>   -thomas
> -- 
> Thomas Lumley
> Professor of Biostatistics
> University of Auckland

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

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