combine -- A simple program to combine n, mean, and SD from two groups according to the Cochrane-recommended formula (for meta-analyses).
combine n1 mean1 sd1 n2 mean2 sd2
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.
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.
combine saves the following in r():
r(n) combined group size r(mean) combined mean r(SD) combined SD
Philip M Jones, MD FRCPC Department of Anesthesiology & Perioperative Medicine Faculty of Medicine & Dentistry University of Western Ontario London, Ontario, Canada firstname.lastname@example.org
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.