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st: RE: Zcalc - with different sd in two groups


From   "Lachenbruch, Peter" <Peter.Lachenbruch@oregonstate.edu>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Zcalc - with different sd in two groups
Date   Thu, 14 Jul 2011 08:39:31 -0700

Why not calculate it from the definition of the z-score?
For common sd you have
Z=(x1bar-x2bar)/s(sqrt(1/n1+1/n2))
For different sd you have
Z=(x1bar-x2bar/sqrt(s1^2/n1+s2^2/n2)
It's a one line program either way.

Tony

Peter A. Lachenbruch
Department of Public Health
Oregon State University
Corvallis, OR 97330
Phone: 541-737-3832
FAX: 541-737-4001

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lars Folkestad
Sent: Thursday, July 14, 2011 4:47 AM
To: statalist@hsphsun2.harvard.edu
Subject: st: Zcalc - with different sd in two groups

Dear Stata list

I have two groups with two means and different standard deviations (SD), and the latter is the problem.
Using the zcalc program, se description below, i can calculate a z-score between the groups if the SD was the same in both groups.

How do i calculate the normal value (z-score) for a measurement method when the groups have different means?

Thank you
Lars

The zcalc option from findit:
---------------------------
Description
-----------

zcalc is a silly little program that computes z-scores.
        If you provide the arguements x mean sd, zcalc
        computes a z-score for observations.  If you
        provide the arguements mean mu sd n in the arguement
        list zclac computes a z-score for sample means.

---------------------------------

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