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st: Effect size and Power for the difference between means


From   Diego Bellavia <messadua@yahoo.it>
To   STATAlist <statalist@hsphsun2.harvard.edu>
Subject   st: Effect size and Power for the difference between means
Date   Wed, 15 Aug 2007 14:47:05 +0000 (GMT)

Dear Statalisters, 

I am sorry for the basic questions, but I have a problem trying to figure out 
how the sizefx command works in STATA 9.2. 

1) Let us say I have two groups  and some outcome variables for which I would 
like to calculate the ES. 
Looking at the help file of sizefx, examples are like 

sizefx age1 age2 
sizefx before after (matched design) 

The problem is that I do not know how to calculate age1 and age2 (I know this is very very basic, 
please mercy on me). 
Then I tryed something like 

sizefx groups age 

to see the ES of age in the different groups, and I got something like 

Effect Size Measures: Cohen's d and Hedges' g
-----------------------------------------------------
Cohen's d statistic (pooled variance) = 2.2798227
Cohen's d statistic (t-statistic method) = 2.2847456
Hedges' g statistic = 2.2847456

Effect size correlations: r
-----------------------------------------------------
ES correlation (Cohen's d method) r = .75173368
ES correlation (t-statistic method) r = .75214284
ES correlation (Hedges' g method) r = .75214284

No problems with the r but regarding the Cohen's d and the Hedge's g 2.27 and 2.28 values
become 0.227 and 0.228 respectively ?  

Another weird thing is that sometimes I have higher ES for variables that are not significantly 
different between the two groups...how is it possibile ? 

 
2) Regarding the sample size calculations for differences between means what command do you reccomend ? 
   I found "sampsi" (t-test) then "fpower" and "simpower" (for ANOVA). Is this a  complete list or do you use something else ? 
    Are there improvements in STATA 10 regarding this ? 


Thank you in advance for the help

Diego


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