help combine
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Title

    combine -- A simple program to combine n, mean, and SD from two groups
           according to the Cochrane-recommended formula (for meta-analyses).

Syntax

        combine n1 mean1 sd1 n2 mean2 sd2


Description

    combine is meant to be used by analysts performing meta-analysis
    whereupon it is sometimes necessary to combine size, mean, and SD from
    two groups (for example, two intervention groups). The Cochrane
    Collaboration provides a formula for this calculation in its handbook,
    but it is cumbersome to perform using a calculator, or by hand.

    combine allows this calculation to be done automatically.


Details on Syntax
    Assume you have two groups:

Group A: n = 50, mean = 26.5, SD = 4.5
Group B: n = 50, mean = 32.9, SD = 5.8

You would type:

combine 50 26.5 4.5 50 32.9 5.8


To then combine a third group (n = 50, mean = 36.4, SD = 3.8), you would type:

combine r(n) r(mean) r(SD) 50 36.4 3.8


Notes
    For the purposes of the combined SD calculation, absolute values of the
    means are used. This is to allow the correct calculation of the combined
    mean between groups (for instance if one or both means were negative
    values) whilst calculating the correct combined SD. (This is specifically
    relevant in the equation for the combined SD that has the specific
    portion "... -2M1M2 ...".) If absolute values of the means were not used
    here, this term would be calculated incorrectly.

References:

    1) Higgins JPT, Green S (editors). Cochrane Handbook for Systematic
    Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane
    Collaboration, 2008.  Available from www.cochrane-handbook.org. The
    specific formula used is in Table 7.7.a.


Saved results

    combine saves the following in r():

    Scalars:            

      r(n)                combined group size
      r(mean)             combined mean
      r(SD)               combined SD


Author Information:

    Philip M Jones, MD FRCPC
    Department of Anesthesiology & Perioperative Medicine
    Faculty of Medicine & Dentistry
    University of Western Ontario
    London, Ontario, Canada
    pjones8@uwo.ca

Change Log:

        1.0.1: 29mar2011 - Updated to notify users about absolute value usage
            (see Notes) and added syntax for combining a third group using
            the returned scalars.

        1.0.0: 28feb2011 - Initial version published.