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From |
"Ariel Linden, DrPH" <ariel.linden@gmail.com> |

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

Subject |
re: Re: st: sampsi and percentages |

Date |
Wed, 31 Aug 2011 09:28:01 -0400 |

I agree with Nick on the need to run simulations. The obvious problem you have is that there are too many moving parts. If you took the mean of the ratio, it would not be accounting for the fact that you could achieve the same ratio by moving the numerator and denominator up or down in opposing directions. Perhaps you could hold one of the variables (numerator or denominator) constant (at the mean or mode, etc.) and run the sample size calculations, then hold the other variable constant and rerun. If you get similar sample sizes, you know that you're not far off. I'd be very cognizant of the standard deviations as well, since I am not sure how large or small they'll be, given that you say the ratios are bounded. Ariel Date: Tue, 30 Aug 2011 20:08:11 +0100 From: Nick Cox <njcoxstata@gmail.com> Subject: Re: st: sampsi and percentages Sure. I was partly in jest, but as a scientist I never feel constrained by the particular units in which data arrive, especially if they are not even metric units, let alone natural. Your example remains units-dependent in that numerator and denominator have quite different units. Not important unless this is also true of your real example. The best way forward for you is likely to be not looking for a canned approach but simulating datasets of different sizes under plausible generating processes and seeing what is or is not detectable. Nick On Tue, Aug 30, 2011 at 7:33 PM, Ricardo Ovaldia <ovaldia@yahoo.com> wrote: > > Thank you Nick. I was specificaly talking about how lenght and weight are recorded in the auto data. > > gen r= length / weight > . sum r > Variable | Obs Mean Std. Dev. Min Max > -------------+-------------------------------------------------------- > r | 74 .0647308 .0102566 .0475524 .0872222 > > The ratio is less that one in all observations! > So the statement was not incorrect. > > Regarding your second point: Yes it behaves as a proportion but I cannot use a sample size calculation for proportion to power this study because there is not a true denominator. > Which brings me back to my original issue of how to power this study. > > Ricardo. > > Ricardo Ovaldia, MS > Statistician > Oklahoma City, OK * * 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/

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