help combine-------------------------------------------------------------------------------

Title

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

Syntax

combinen1 mean1 sd1 n2 mean2 sd2

Description

combineis 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.

combineallows this calculation to be done automatically.

Assume you have two groups:Details on SyntaxGroup 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

NotesFor the purposes of the combined SD calculation, absolute values of themeans are used. This is to allow the correct calculation of the combinedmean between groups (for instance if one or both means were negativevalues) whilst calculating the correct combined SD. (This is specificallyrelevant in the equation for the combined SD that has the specificportion "... -2M1M2 ...".) If absolute values of the means were not usedhere, this term would be calculated incorrectly.

References:

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

Saved results

combinesaves the following inr():

Scalars:

r(n)combined group sizer(mean)combined meanr(SD)combined SD

Author Information:

Philip M Jones, MD FRCPCDepartment of Anesthesiology & Perioperative MedicineFaculty of Medicine & DentistryUniversity of Western OntarioLondon, Ontario, Canadapjones8@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 usingthe returned scalars.

1.0.0:28feb2011 - Initial version published.