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From |
"Don Spady" <dspady@ualberta.ca> |

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

Subject |
st: Re: RE: Re: RE: Sample size |

Date |
Mon, 28 Apr 2003 11:52:49 -0600 |

Paul Thank you for your help. I will give it a try. Don ----- Original Message ----- From: "VISINTAINER PAUL" <VISINT@NYMC.EDU> To: <statalist@hsphsun2.harvard.edu> Sent: Monday, April 28, 2003 11:35 Subject: st: RE: Re: RE: Sample size > The formula for estimating the sample size based on the width of a > confidence interval for a proportion is: > > n = (z^2 * p * q)/(d)^2, where z is the alpha level and d is the one-sided > difference between p and the upper (or lower) limit. For example, if you > expect a proportion of .35, and you want to be 95% sure that p is no larger > than .40 (given that p is .35). So with z=1.96, p=.35, q=.65, and d=(.40 - > .35) or .05, your sample size is about 350. > > Interestingly, you can get this using the -cii- command. > > .cii 350 .35, the difference though is that these confidence intervals are > exact. > > In your case, I wouldn't use the normal approximation to the binomial > because your proportion is quite rare. You could use -cii- and try > different sample sizes, while maintaining the same proportion, e.g., > > .cii 1000 1, will give a wide confidence interval > > .cii 10000 10, will get you a narrower one. > > (Note these are exact limits, not normal approximations) > > What sampsi does is model both alpha error and beta error. As long as you > don't specify a value for an alternative hypothesis (i.e., all you are > interested in is interval estimation) you don't need to model beta error. > > > > Paul > > > > > -----Original Message----- > From: Don Spady [mailto:dspady@ualberta.ca] > Sent: Monday, April 28, 2003 1:07 PM > To: statalist@hsphsun2.harvard.edu > Subject: st: Re: RE: Sample size > > Paul > Thanks for your reply. Indeed I want to estimate prevalence, with > the interval being from 0.0001 to 0.0003 or there abouts. I was told > that the prevalence of the disease was between 1:200 and 1:2000, > possibly closer to the 1:200. By shooting at 0.0005, I would get the > worst case scenario. The confidence interval is hard to guess (say the > real value is 1:200 and I test for 1:2000, how do I estimate a > confidence interval) If the presence or absence follows a poisson > distribution, then the variance is 1:2000 and the SD is 0.0224, I think. > Does this make much sense. > > Don > > > ----- Original Message ----- > From: "VISINTAINER PAUL" <VISINT@NYMC.EDU> > To: <statalist@hsphsun2.harvard.edu> > Sent: Monday, April 28, 2003 10:09 > Subject: st: RE: Sample size > > > > 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/ > > > > > * > * 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/ > * * 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/

**References**:**st: RE: Re: RE: Sample size***From:*VISINTAINER PAUL <VISINT@NYMC.EDU>

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