[R] bias sampling

Thomas Lumley tlumley at uw.edu
Sun Mar 18 20:54:30 CET 2012

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.

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.


Thomas Lumley
Professor of Biostatistics
University of Auckland

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