# st: Effect size and Power for the difference between means

 From Diego Bellavia To STATAlist 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,
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

___________________________________
L'email della prossima generazione? Puoi averla con la nuova Yahoo! Mail: http://it.docs.yahoo.com/nowyoucan.html

*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/
```