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Re: st: Test for significance of the difference between knee locations?

From   "Roger B. Newson" <>
Subject   Re: st: Test for significance of the difference between knee locations?
Date   Thu, 18 Oct 2012 21:36:14 +0100

What is the "knee" of a quadratic model? I never knew that they had knees (although I might expect a 3-parameter linear spline to have a knee).

Best wishes


Roger B Newson BSc MSc DPhil
Lecturer in Medical Statistics
Respiratory Epidemiology and Public Health Group
National Heart and Lung Institute
Imperial College London
Royal Brompton Campus
Room 33, Emmanuel Kaye Building
1B Manresa Road
London SW3 6LR
Tel: +44 (0)20 7352 8121 ext 3381
Fax: +44 (0)20 7351 8322
Web page:
Departmental Web page:

Opinions expressed are those of the author, not of the institution.

On 18/10/2012 21:25, Jordan Silberman wrote:
Hi Stata folks,

I'm searching for a method I can run in Stata that will allow me to
test whether the location of the knee for one quadratic model
significantly differs from that of another quadratic model.

I have 2 quadratic models, each modeling a different category of
healthcare costs as a function of age. Let's say, for illustrative
purposes, that the knee of the first quadratic model occurs at age 45,
and the knee for the second model occurs at age 55.

Can anybody suggest a method for testing whether the former "knee age"
differs significantly from the latter "knee age?" The idea is to test
the hypothesis that there is a difference across the 2 models in the
age at which the costs growth rate begins to increase.

Any suggestions would be greatly appreciated.

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