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From |
VISINTAINER PAUL <VISINT@NYMC.EDU> |

To |
"'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: Sample size |

Date |
Mon, 28 Apr 2003 12:09:57 -0400 |

Don, The problem you are having with sample size is that you haven't given enough information. It isn't clear whether you want to simply estimate the prevalence/incidence of a condition in the population; whether you want to "test" whether the occurrence in the population is really .001, or whether you want to test the difference between groups, assuming the occurrence in general is .001. The last two options require you to specify an alternative hypothesis, which you haven't given. Using your sampsi input, you are specifying a comparison between a prevalence of 1 per 1000 vs. none (or a really very, very rare prevalence). In this case you're specifying that the null value is .001 and your alternative is that it is much more rare than that. If you reverse your figures (e.g., sampsi 0 .001, p(.8)) you're specifying that the null value is near 0 and your alternative hypothesis is that it is much more prevalent. (I was actually surprised that sampsi performed the calculation with 0 as an entry. I suppose it actually uses a very small value for 0.) For the first option, you rather just estimate the prevalence of this condition, (which you think is pretty rare at .001), you might want to focus on the precision of the estimate by specifying the width of the confidence interval. I don't think we can get a sample size estimate based on the width of a confidence interval using sampsi. So, what do you want to do? Paul -----Original Message----- From: Don Spady [mailto:dspady@ualberta.ca] Sent: Monday, April 28, 2003 11:19 AM To: Statalist Subject: st: Sample size Dear all I sent this before but got no response. I have revised it. I want to estimate the sample size needed to detect an disease that occurs in 1 out of 1000 people (as an example). The alternate state is absence of disease which would occur in 999 of 1000 people on average. The problem is that I get numbers but I don't know if they are the right ones. Can I use sampsi grp1 being those with disease and Grp2 being those without disease. Or do I use sampsi 0.001, onesample as in: sampsi 0.001 0, p(0.8) onesample I need help and thank in advance those that provide it. Donald Spady Dep't of Pediatrics, University of Alberta (780) 407-1244 Nature has no reset button. Donald Spady Dep't of Pediatrics, University of Alberta (780) 407-1244 Nature has no reset button. * * 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/ * * 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**:**st: Re: RE: Sample size***From:*"Don Spady" <dspady@ualberta.ca>

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