Diego,
My impression, given what you say in your message, is that you may have a
misunderstanding of what -sizefx- is intended to compute.
Generally, an effect size is a mean difference divided by the standard error
of that difference (or, alternatively, is the correlation of two variables
divided by the standard error of that correlation). This has nothing to do
with computing sample size, power, etc.
-sizefx age1 age2- merely computes the mean, variance and sample size for
each variable (age1, age2) then computes the various effect sizes.
For example, Cohen's d is computed as:
d = abs(m1-m2) / sqrt(((n1*v1)+(n2*v2))/(n1+n2))
where m1 is the mean, n1 is the sample size and v1 is the variance of age1
and m2 is the mean, n2 is the sample size and v2 is the variance of age2
d is then read to indicate that the means of age1 and age2 differ by d
standard deviations.
Clearly, you do not calculate the variables age1 or age2, you just use them
in the program.
It is probably fair to note that the typical use of effect sizes is when
t-values, correlations or the sufficient statistics are all you have (i.e.,
you do not have raw data). Then you just directly plug in what you have into
an appropriate formula and spit out the effect size. Because of this, I do
not see -sizefx- as a particularly useful program.
Tom
-----------------------------------
Thomas J. Steichen
[email protected]
-----------------------------------
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Diego Bellavia
Sent: Wednesday, August 15, 2007 10:47 AM
To: STATAlist
Subject: st: Effect size and Power for the difference between means
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
___________________________________
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/
-----------------------------------------
CONFIDENTIALITY NOTE: This e-mail message, including any
attachment(s), contains information that may be confidential,
protected by the attorney-client or other legal privileges, and/or
proprietary non-public information. If you are not an intended
recipient of this message or an authorized assistant to an intended
recipient, please notify the sender by replying to this message and
then delete it from your system. Use, dissemination, distribution,
or reproduction of this message and/or any of its attachments (if
any) by unintended recipients is not authorized and may be
unlawful.
*
* 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/
