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# Re: st: "dependent correlation" or c-statistics

 From Melaku Fekadu To statalist@hsphsun2.harvard.edu Subject Re: st: "dependent correlation" or c-statistics Date Wed, 18 Aug 2010 17:01:14 +0300

```Dear Mr. Roger,

Thanks you. it was a great help.

Melaku

On Wed, Aug 18, 2010 at 4:04 PM, Roger Newson <r.newson@imperial.ac.uk> wrote:
> It is probably not a good idea to choose a threshold according to whether
> using that threshold gets a P-value of 0.05 or less. And it might not be a
> good idea to use a threshold at all, as doing this implies that information
> about trend within the 2 binary values is not used.
>
> To measure ordinal trend, without converting -ves- to binary, you might like
> to use the -somersd- package, downloadable from SSC (or from my website if
> you use Stata 9). If you install this package, then you can type
>
> somersd frailty ves, transf(z) tdist
>
> and you will compute a confidence interval for Somers' D of -ves- with
> respect to -frailty-. And you can type
>
> somersd ves frailty, transf(z) tdist
>
> and compute a confidence interval for the Somers' D of -frailty- with
> respect to -ves-.
>
> Alternatively, you can use the Harrell's c parameterization. Harrell's c is
> linked to Somers' D by the relation D=2c-1, where D is Somers' D and c is
> Harrell's c. To calculate a confidence interval and P-value for Harrell's c
> of -ves- with respect to -frailty-, type
>
>
> somersd frailty ves, transf(c) tdist
>
> and to calculate a confidence interval and a P-value for Harrell's c of
> -frailty- with respect to -ves-, type
>
> somersd ves frailty, transf(c) tdist
>
> I hope this helps.
>
> Best wishes
>
> Roger
>
>
> 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
> UNITED KINGDOM
> Tel: +44 (0)20 7352 8121 ext 3381
> Fax: +44 (0)20 7351 8322
> Email: r.newson@imperial.ac.uk
> Web page: http://www.imperial.ac.uk/nhli/r.newson/
> Departmental Web page:
>
> Opinions expressed are those of the author, not of the institution.
>
> On 18/08/2010 12:34, Melaku Fekadu wrote:
>>
>> hello statalisters,
>>
>> i have two variables: one called 'ves' with values between 1 to 10,
>> and the other is dichotomus called 'frality' with value either 0 or 1.
>>
>> i create a categorical variable from 'ves' 0-2 and 3-10 and make a
>> chi-square test to test a relation with the variable frality, and i
>> get p>0.05. But when i create another categorical variable of 'ves'
>> with 0-4 and 5-10 and do the chi-square test i find that p<0.05.
>>
>> my question: what can i do to get a threshold value of 'ves' where
>> p-value will be 0.05? somebody told me about a procedure calles
>> "dependent correlation" or c-statistics in stata.
>>
>> any help will be appreciated.
>>
>> thanks
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```