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Re: st: Average age at first occurrence analysis

From   Steven Samuels <>
Subject   Re: st: Average age at first occurrence analysis
Date   Thu, 12 Feb 2009 13:34:26 -0500



I'm not familiar with the R package models, and I don't know what they do. In Stata, the Weibull model is available in -streg-. The Cox model can be considered a generalization: both are proportional hazard models; in the Weibull, there is a parametric form for the hazard function; in the Cox model there is not. As Greenland and Boshuizen show, the PH assumption is not necessary to an age-at-onset analysis. One can compute the difference between portions of any two survival curves. If the Weibull assumption does hold, then one can do it parametrically. Obviously the caveats that Wilk and Lash raise apply.

The two articles you referred to are both available online.    Wilk and Lash Greenland and Boshuizen



On Feb 11, 2009, at 7:46 PM, Andrew John Brunskill wrote:

Please would any kind people suggest some stata commands that would allow me to calculate estimated average times , using Weibull type models, with particular emphasis on their use in case only designs. The only reference in the stata list I can find is on the shared frailty model but most of the sources I can find are on the Cox (proportional hazards) model. There was a very valuable paper by Boshuizen HC and Greenland S Int J Epidemiology 1997 26 4 867 âAverage age at first occurrence as an alternative occurrence parameterâ which âencouraged the development of suitable packaged software allowing the fitting of these modelsâ and an excellent but scary paper by Wilk JB and Lash TL Emerging Themes in Epidemiology 2007 4:1 âRisk factors studies of age-at-onset..:a cautionary taleâ which shows the methodological and interpretative problems. Prof Greenland has been kind enough to clarify for me what I need to understand. What I would love to get as a reasonably numerate medical epidemiologist ( but not someone who models bus waiting times in greek numerals in my head (alas!) )is a good book or source (lecture notes??) with some worked commands in hopefully stata. My work is on data on kidney failure and transplant so it might even be helpful to some other people sometime! Thanks very much for any replies. Andrew Brunskill

Andrew J Brunskill,
University of Washington
Fellow in Epidemiology ANZDATA

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