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# Re: st: obtain SD from P value for a paired t-test

 From Steve Samuels <[email protected]> To [email protected] Subject Re: st: obtain SD from P value for a paired t-test Date Tue, 1 Oct 2013 18:37:59 -0400

```Marcos:

You can't get the SD post without knowing the correlation of the pre & post measurements,
since:

sd_diff = sqrt ( sd_pre^2 + sd_post^2  - 2 rho sd_pre sd_post)

Steve
On Oct 1, 2013, at 1:56 PM, Marcos Vinicius wrote:

better formulating the question :obtaining the SD of the post measure
using the   information bellow and additionally we know the N for the paired t test.

Vinicius
-----Original Message-----
From: [email protected]
Sent: Tue, 1 Oct 2013 09:48:14 -0800
To: [email protected]
Subject: Re: st: obtain SD from P value for a paired t-test

Thanks Rich but not solved yet.
I know the N,so I can get the standard deviation of the diference(post-pre).
However I still don t know if it is possible to obtain the standard deviation corresponding to the post measure.
Vinicius
> -----Original Message-----
> From: [email protected]
> Sent: Tue, 01 Oct 2013 13:04:47 -0400
> To: [email protected]
> Subject: Re: st: obtain SD from P value for a paired t-test
>
> if you know N also, you can use the formula given in the Stata manual
> (methods and formulas section of the ttest entry in the reference
> manual), or many texts, to get the sd - if you don't know N, the best
> you can do is an approximation
>
> Rich
>
> On 10/1/13 12:09 PM, Marcos Vinicius wrote:
>> Hello,
>> Anyone knows a way to obtain  standard deviation(sd) from P value for a
>> paired t-test.?
>> Consider that I know:
>> Average pre
>> SD pre
>> Average post
>> SD post (to be determined from others parameters)
>> P value from paired t test
>>
>> Average pre=6.5
>> SD pre=1.8
>> Average post =4.9
>> SD post ??(to be determined from others parameters)
>> P value from paired t test p=0.0003
>> Any help would be greatly appreciated
>> Thanks
>> Vinicius
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```