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From |
Joseph Coveney <jcoveney@bigplanet.com> |

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

Subject |
st: Re: sample size calculation |

Date |
Thu, 30 Sep 2004 12:28:31 +0900 |

It seems as Ricardo's authors are assaying nicotine or its metabolites in some tissue (placenta, perhaps). In pharmacokinetics, variation often *is* expressed in terms of the coefficient of variation, as Clint suggests. If Ricardo's authors intended to say that the standard deviation is 15% instead of 15 ng/mg, and if they contemplated using Bonferroni's inequality to adjust alpha for two comparisons among the three groups, then -sampsi 100 115, alpha(0.025) power(0.8) sd1(15) sd2(15)- gives 43. Here, I'm assuming that power, not beta, is 80%. It's possible that Ricardo's authors' article is peppered with unintentional errors and omissions in describing things, such as power and the scale for variation. In planning clinical studies, it's common to supplement the estimated sample size in anticipation of participants being lost to follow-up or withdrawing informed consent, and in anticipation of those whose data might be considered ineligible for analysis due to after-the-fact discovery of protocol violations, and so on. With such compensation for potential drop-out cases, the sample size could have been increased to 50, or so, for each group. Joseph Coveney -------------------------------------------------------------------------------- Clint Thompson wrote: I'm no authority, but there is a small section in Van Belle's "Statistical Rules of Thumb" that discusses sample size calculations in terms of relative changes, e.g. percent. Van Belle frames the sample size calculation in terms of 'proportionate change in means' and 'coefficient of variation' which would require a percent expression of the variability. If possible, you may want to consider reading Van Belle's discussion on this topic. --Clint >>> ovaldia@yahoo.com 9/29/2004 12:04:04 PM >>> Dear all, In an article that I am reading the authors write: "it was decided a priori to include a minimum quantity of mother, which could allow the statistical power (alpha=0.05, beta=0.80) for assessing differences of 10% in nicotine concentration by group with an expected standard deviation of 15 ng/mg. These criteria led to the analysis of 150 mothers (50 for each of the 3 groups)" They claim that they did this and all analysis using Stata 5.0. They do not provide group means or any discriptive statistics on which the sample size was computed. Can someone please tell me how this could have been done. And is there a program to do this in Stata? Thank you in advance, Ricardo * * 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: Re: sample size calculation***From:*Richard Williams <Richard.A.Williams.5@nd.edu>

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