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

 From Yuval Arbel To statalist@hsphsun2.harvard.edu Subject Re: st: Test for significance of the difference between knee locations? Date Thu, 18 Oct 2012 22:50:55 +0200

```I believe Jordan's intention is literally the knees in the human body,
and not to a mathematical or statistical concept

On Thu, Oct 18, 2012 at 10:47 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> 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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--
Dr. Yuval Arbel