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From |
Ricardo Ovaldia <ovaldia@yahoo.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Sample size for ttest - (am I wrong?) |

Date |
Tue, 18 Feb 2003 13:44:39 -0800 (PST) |

Thank you David and Ronan. I'm confused by the apparent contradictions in the two posts. I understood how Ronan calculated the power but its this valid? Or is the problem (as I originally suspected) that a "difference of 1.5 SD" doesn't mean anything without knowing something about the distribution of the two groups? Am I missing something? Thanks in advance, Ricardo. --- David Moore <davem@hartman-group.com> wrote: > Ricardo, > > You're correct, but the "other" statistician > probably made a number of > assumptions to fill in the blanks, so your colleague > wouldn't be bothered > with having to think about the substantive issues. > I did statistical > consulting for several years and this (power > analysis) was one of many > problems that I had to spend a lot of time > explaining to clients. Most of > the time I ended up walking them through sections of > Cohen's seminal book > just to save myself from going insane. I suspect > the pathologist's friend > was using Cohen's (cook)book or a software package > based on Cohen's formulas > and suggestions. If he was a true friend, he would > have spent the time > explaining the underlying logic to the pathologist > and he (the pathologist) > would have been able to speak intelligently about > his expectations for the > means and standard deviations. > > It's been my experience that many experienced and > fairly competent > researchers do not really conduct a proper power > analysis, when it's called > for. Your experience with the pathologist is a > prime example of the > investigator proceeding without having a full > understanding of the problem > at hand. In my opinion, if he doesn't have a clue > about the effect size of > interest, he shouldn't be doing the study. > > Dave --- Ronan Conroy <rconroy@rcsi.ie> wrote: > on 18/02/2003 5:11 pm, Ricardo Ovaldia at > ovaldia@yahoo.com wrote: > > > and furthermore I did > > not need this information because his friend > (another > > "statistician") told him that with 8 observations > per > > group he could detect a difference of 1.5 standard > > deviations with 80% power. > > . sampsi 0 1.5, sd(1) n(8) > > Estimated power for two-sample comparison of means > > Test Ho: m1 = m2, where m1 is the mean in population > 1 > and m2 is the mean in population > 2 > Assumptions: > > alpha = 0.0500 (two-sided) > m1 = 0 > m2 = 1.5 > sd1 = 1 > sd2 = 1 > sample size n1 = 8 > n2 = 8 > n2/n1 = 1.00 > > Estimated power: > > power = 0.8508 > > A different of 1.5 standard deviations is very large > indeed. Yes, the power > is 85%, but the Mann Whitney statistic is 86% > > > . mwstati 1.5 1 1, i > Mann-Whitney statistic for this situation is: 0.8556 > > That is, the probability that an observation in > group 2 will be higher than > an observation in group 1 is 0.8556. > > Two comments: > 1. The minimum clinically significant difference in > Cox2 may well be less > than 1.5 SD and > 2. An 80% power is an unacceptably high risk of > failure. I don't get out of > bed for less than 90%. > > > Ronan M Conroy (rconroy@rcsi.ie) > Lecturer in Biostatistics > Royal College of Surgeons > Dublin 2, Ireland > +353 1 402 2431 (fax 2764) > > -------------------- > > * > * 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/ ===== Ricardo Ovaldia, MS Statistician Oklahoma City, OK __________________________________________________ Do you Yahoo!? Yahoo! Shopping - Send Flowers for Valentine's Day http://shopping.yahoo.com * * 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/

**Follow-Ups**:**Re: st: Sample size for ttest - (am I wrong?)***From:*Ronan Conroy <rconroy@rcsi.ie>

**References**:**Re: st: Sample size for ttest - (am I wrong?)***From:*Ronan Conroy <rconroy@rcsi.ie>

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