```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