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From |
Steven Samuels <[email protected]> |

To |
Steven Samuels <[email protected]> |

Subject |
Re: st: Wald interval and the WSJ |

Date |
Thu, 14 Aug 2008 13:39:23 -0400 |

I misnumbered the probabilities in the 2nd paragraph and probably confused everyone. The paragraph should have been: With the article data. P0 = .062 and P1 = .092. The denominator term in the test statistic for the Wald test is 0.012401. The denominator term with P0 and P1 is .012591. The ratio is .984876. Therefore the proper Z statistic would be equal to the Wald statistic reduced by this ratio. -Steve On Aug 14, 2008, at 12:35 PM, Steven Samuels wrote: > The 1-sided .95 confidence interval for the treatment difference > given in the article excludes the null value of .03; the problem is > that the proper non-inferiority test statistic would have p>.05. > Fleiss, Levin, Paik (Statistical Methods for Rates and Propotions, > 2nd Ed, Wiley, pp 168-174) show one proper non-inferiority > statistic (I'm not sure which of the listed alternatives it > corresponds to). In an ordinary Wald Z Statistic, the denominator > contains terms in p1(1-p1)/n1 and p0(1-p0)/n0. In the non- > inferiority setting, the probabilities are changed to: P1 and P0, > where P1 - P0 =.03 (the null hypothesis. These are the maximum > likelihood estimates under the null hypothesis. > > With the article data. P1 = .062 and P2 = .092. The denominator > term in the test statistic for the Wald test is 0.012401. The > denominator term with P1 and P2 is .012591. The ratio is . > 984876. Therefore the proper Z statistic would be equal to the > Wald statistic reduced by this ratio. > > Now, the p-value for the Wald statistic in the paper was .0487, > equivalent to Z = 1.6575912. The "proper z" would have been > 1.6325, with a p-value of 0.0512, close to what the WSJ article > reported for the alternatives. > > -Steve > > On Aug 14, 2008, at 11:08 AM, Maarten buis wrote: > >> --- David Airey <[email protected]> wrote: >>> And also the article doesn't emphasize effect size, which might make >>> the quibbling over p values moot too. >> >> Actually, the point estimate seems to suggest that the new stent does >> better then the old one, if I read the original article >> (http://content.onlinejacc.org/cgi/content/full/49/16/1676) >> correctly, >> and I know absolutely nothing about cardiology other than that a >> working hart is sorta crucial in staying alive. >> >> I read this article as follows: The thing under study is how well two >> types of stents prevent a thing called TVR (target vessel >> revascularization) which is apperently a bad thing. In the group with >> old stents this occured 68 times out of 855 (7.95%), while in the >> group >> with new stents this occured 67 out of 956 times (7.01%). The purpose >> of this study is to test the hypothesis that the proportion new - >> porportion old > .03 . >> >> I have been looking around if I could reproduce the tests reported in >> the Wall Street Journal using Stata, but remained unsuccesful. This >> probably says more about me than about Stata, because statistically >> this is also unfamiliar terain for me, as I almost always do some >> sort >> of regression, moreover if I where to look at tests like these I >> would >> probably prefer odds ratios rather than risk differences. >> >> -- Maarten >> >> ----------------------------------------- >> Maarten L. Buis >> Department of Social Research Methodology >> Vrije Universiteit Amsterdam >> Boelelaan 1081 >> 1081 HV Amsterdam >> The Netherlands >> >> visiting address: >> Buitenveldertselaan 3 (Metropolitan), room Z434 >> >> +31 20 5986715 >> >> http://home.fsw.vu.nl/m.buis/ >> ----------------------------------------- >> >> Send instant messages to your online friends http:// >> uk.messenger.yahoo.com >> * >> * 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/ > Steven Samuels 845-246-0774 18 Cantine's Island Saugerties, NY 12477 EFax: 208-498-7441 * * 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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