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

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

Subject |
Re: st: sampsi and percentages |

Date |
Tue, 30 Aug 2011 11:33:40 -0700 (PDT) |

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 ----- Original Message ----- From: Nick Cox <njcoxstata@gmail.com> To: statalist@hsphsun2.harvard.edu Cc: Sent: Tuesday, August 30, 2011 10:02 AM Subject: Re: st: sampsi and percentages Two points: 1. In terms of your example, length/weight is not always < 1. The value of that ratio is crucially dependent on some choice of units of measurement. Suppose I measure my (wife's) car's length in centimetres and its weight (mass) in tonnes, for example. You can call this pedantry but I react to incorrect statements! 2. More importantly, if something is bounded by (0,1) -- can we take that pair of () literally as implying 0 < data < 1? -- then it will behave like a proportion regardless of how the calculation was done. For example, an average very near 0 can only be achieved if all values are near 0 and so the variance will be very small, and similarly for an average near 1. However, that may not help much. Nick On Tue, Aug 30, 2011 at 3:45 PM, Ricardo Ovaldia <ovaldia@yahoo.com> wrote: > > Thank you, but these are not proportions. They are intensity measures. You can think of them as ratios of two continous things. > For example with the auto data, they could be the ratio of car's length to weight (length / weight) which is always between 0 and 1. > Now less say that you want to compare these ratio between between foreign and domestic cars. > > Ricardo > > Ricardo Ovaldia, MS > Statistician > Oklahoma City, OK > > > ----- Original Message ----- > From: "Ariel Linden, DrPH" <ariel.linden@gmail.com> > To: statalist@hsphsun2.harvard.edu > Cc: > Sent: Tuesday, August 30, 2011 7:51 AM > Subject: re: st: sampsi and percentages > > Ricardo, > > I may be mistaken here, but it seems you have two proportions (if it's > bounded between 0,1 then you have a numerator and a denominator for each > group). > > If that is truly the case, you can use sampsi for proportions: > > . sampsi 0.25 0.4 > > Estimated sample size for two-sample comparison of proportions > > Test Ho: p1 = p2, where p1 is the proportion in population 1 > and p2 is the proportion in population 2 > Assumptions: > > alpha = 0.0500 (two-sided) > power = 0.9000 > p1 = 0.2500 > p2 = 0.4000 > n2/n1 = 1.00 > > Estimated required sample sizes: > > n1 = 216 > n2 = 216 > > I hope this helps > > Ariel > > Date: Mon, 29 Aug 2011 11:40:33 -0700 (PDT) > From: Ricardo Ovaldia <ovaldia@yahoo.com> > Subject: st: sampsi and percentages > > > > I need to compute sample size and power for a study comparing two group on a > measurement bounded by (0,1), (a measure of intensity). > I was thinking about using -sampsi- to power on the difference of means. > However, this seems strange to me, is there another way to power such > comparison? > > Thank you, > 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/ > > > * > * 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/ > * * 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/ * * 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**:**Re: st: sampsi and percentages***From:*Nick Cox <njcoxstata@gmail.com>

**References**:**re: st: sampsi and percentages***From:*"Ariel Linden, DrPH" <ariel.linden@gmail.com>

**Re: st: sampsi and percentages***From:*Ricardo Ovaldia <ovaldia@yahoo.com>

**Re: st: sampsi and percentages***From:*Nick Cox <njcoxstata@gmail.com>

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