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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