 |
 |
Dear Giuseppe, The question of CIs for LRs has come up several times before on the list. I don't know if any of these articles or softwares calculate power and sample sizes; however, with an equation set up in a spread sheet or with off-the-shelf software you can simply plug in sample sizes until you find what gives you an acceptable CI by trial and error. Here are LR CI methods from the previous postings: Simel DL, Samsa GP, Matchar DB. Likelihood ratios with confidence: sample size estimation for diagnostic test studies. Journal of Clinical Epidemiology 1991; 44:763-770. You can find a Bayesian clinical calculator at: http://www.healthcare.ubc.ca/calc/bayes.html Statsdirect calculates LR CIs. http://www.statsdirect.com/update.htm or WWW.STATSDIRECT.COM I think there is a free sample there. "you might want to check the statistics appendix section in Sackett's latest book: Evidence-based Medicine: how to practise and teach: publ Churchill Livingstone 2000. There's an equation that allows you to calculate LR confidence intervals." However, there is apparently an error in that method. But a correction has been published. "The correction of the equation from the EBM book is posted on the university of toronto website (see erratum 21 at http://www.library.utoronto.ca/medicine/ebm/search.htm)" Apologies for quoting other people's e-mails without credit. Something here should work for you. David L. Doggett, Ph.D. Senior Medical Research Analyst Health Technology Assessment and Information Services ECRI, a non-profit health services research organization 5200 Butler Pike Plymouth Meeting, Pennsylvania 19462, U.S.A. Phone: (610) 825-6000 x5509 FAX: (610) 834-1275 http://www.ecri.org e-mail: [log in to unmask] -----Original Message----- From: Dr. Giuseppe Giocoli [mailto:[log in to unmask]] Sent: Thursday, August 23, 2001 6:46 AM To: [log in to unmask] Subject: CIs of LRs Dear List Members, Is there any practical way to calculate a suitable sample size for confidence intervals of Likelihood ratios? Thanks. Giuseppe Dr. Giuseppe Giocoli Gruppo di Lavoro EBM Associazione Microbiologi Clinici Italiani Via Sarca, 19 25015 DESENZANO d/G (BS) Italia