Re: st: metan question,

[Michael McCulloch <mm@pinestreetfoundation.org>]

Thank you Tiago, this is very helpful.
Michael

On Sep 8, 2010, at 10:47 PM, Tiago V. Pereira wrote:

> Michael,
>
> I believe you need further information on the data you have at hand:
> either the t statistic or a correly reported P-value, say, P = 0.052.
> Once you have one of these pieces of information, you can compute an
> approximated value for the variance for your effect size (the  
> difference
> between "after" and "before").  See the formulations in classic
> statistical books.
>
> Denote the effect size as ES, the standard deviation as SD and the  
> sample
> size as N.
>
> So, you end up with an N1, ES1 and SD1 for group 1 and N2, ES2 and  
> SD2 for
> group 2, the exact information required by -metan-.
>
> If you don't have these information, I believe you have two  
> strategies:
>
> Assuming you have the following data for group 1:
>
> N1 = number of subjects (identical in the "before" and "after")
> m1 = mean before intervention
> m2 = mean after intervention
> sd1 = standard deviation before intervention
> sd2 = standard deviation after intervention
>
>
> You can compute the effect size "after vs before" as:
>
>
> ES1 = m2-m1
>
> And an approximate standard deviation for ES1:
>
> SD1 = sqrt(((sd1^2)/(N1))+((sd2^2)/(N1))-(2*sd1*sd2*rho)/ 
> N1)*(sqrt(N1))
>
> where rho is the correlation between observations from m2 and m1.
>
> However, one does not know the correlation from summary data.   
> hence, your
> two strategies are:
>
> (1) set rho = 0, and ignore the correlation. This reduces the power  
> of the
> meta-analysis when rho>0.
>
> (2) set several values of rho and check how results change. these  
> values
> can be obtained from some studies that you have raw data, or from
> literature.
>
> Let me know if that works for you.
>
> Cheers!
>
> Tiago
>
>
>
>
>
>
> ----------------------------------------
>
> Michael wrote:
>
> ello,
> I notice that -metan- requires the following for continuous data:
> 	sample size,
> 	mean difference, and
> 	standard deviation (SD)
>
> of the experimental group followed by those of the control group. This
> could be used, for example, to pool data such as mean difference in  
> blood
> pressure before and after an intervention.
>
> However, if instead of the sample size, mean difference and standard
> deviation, I had for both groups:
>
> 	sample size,
> 	mean blood pressure before intervention
> 	standard deviation (SD)
>
> 	sample size,
> 	mean blood pressure after intervention
> 	standard deviation (SD)
> would it be possible to convert into the format required by -metan-?
>
>
> Best wishes,
>
> Michael McCulloch, LAc MPH PhD
> Pine Street Foundation
> 124 Pine Street
> San Anselmo, CA 94960-2674
> tel:	415-407-1357
> fax: 	206-338-2391
>
>
>
>
>
> *
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Best wishes,

Michael McCulloch, LAc MPH PhD
Pine Street Foundation
124 Pine Street
San Anselmo, CA 94960-2674
tel:	415-407-1357
fax: 	206-338-2391

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/