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

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: Test for significance of the difference between knee locations? Date Thu, 18 Oct 2012 21:47:12 +0100

```I think the only knees here are the positions of the turning-points
(whether maxima or minima) of the quadratics.

But Yuval's suggestion of putting the two models together in one seems
perfectly reasonable.

Nick

On Thu, Oct 18, 2012 at 9:42 PM, Yuval Arbel <yuval.arbel@gmail.com> wrote:
> You can run the following model:
>
> Cost=beta0xleft+beta1xAgexleft+beta2xAge_sqxleft+gamma0xRight+gamma1xAgexRight+gamma2xAge_sqxRight+u
>
> where Cost is the medical cost, Right (Left) equals 1 for the right
> (left) knee and 0 for the left (right) knee, and Age is the patient's
> age.
>
> You now need to test beta0=gamma0, beta1=gamma1 and beta2=gamma2 by
> the -test- command
>
> On Thu, Oct 18, 2012 at 10:25 PM, Jordan Silberman
> <silberman.stata@gmail.com> 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.
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