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From |
Philip Ender <ender97@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: Re: fpower |

Date |
Sat, 20 Jun 2009 20:55:42 -0700 |

"CHIDLOW Agnieszka" <A.Chidlow@staffs.ac.uk> wrote: Please forgive me if you find this request silly (or basic), but I have been having some nightmares to figure out how to use ex post "fpower" for anova. I have read about the power analysis (e.g. papers, and on www.ats.ucla.edu), but it looks like I can't do the test because I don't seem to know how to calculate delta and/or means for my groups. Hence would really appreciate some help from you. My anova results are: (...) --------------------------------------------- "Martin Weiss" <martin.weiss1@gmx.de> replied: Seems from the explanations at http://www.ats.ucla.edu/stat/stata/dae/fpower.htm that these values are mere educated guesses, not results of calculations... ---------------------------------------------- There certainly are educated guesses involved in any power analysis. In the case of -fpower- the main educated guess is concerning the delta(). After that -fpower- computes power based on the noncentral F-distribution. So there are indeed calculations going on. The -simpower- program generates random samples with given means and standard deviations and computes power by counting the number of significant results. I usually suggest at least 1,000 iterations. As for Agnieszka power question, I would recommend running the following: "simpower nm year, reps(2000)" I'm a little confused by the statement that nm is 2 groups. Does this mean that there is a categorical response variable? If so, then -anova- is the wrong tool to use. -- Phil Ender Statistical Consulting Group UCLA Academic Technology Services * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**st: fpower***From:*"CHIDLOW Agnieszka" <A.Chidlow@staffs.ac.uk>

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