title: Confidence intervals for a difference between lognormal means in cluster randomization trials.
abstract: Cluster randomization trials, in which intact social units are randomized to different interventions, have become popular in the last 25 years. Outcomes from these trials in many cases are positively skewed, following approximately lognormal distributions. When inference is focused on the difference between treatment arm arithmetic means, existent confidence interval procedures either make restricting assumptions or are complex to implement. We approach this problem by assuming log-transformed outcomes from each treatment arm follow a one-way random effects model. The treatment arm means are functions of multiple parameters for which separate confidence intervals are readily available, suggesting that the method of variance estimates recovery may be applied to obtain closed-form confidence intervals. A simulation study showed that this simple approach performs well in small sample sizes in terms of empirical coverage, relatively balanced tail errors, and interval widths as compared to existing methods. The methods are illustrated using data arising from a cluster randomization trial investigating a critical pathway for the treatment of community acquired pneumonia.
authors.affiliation: 1 Department of Epidemiology & Biostatistics, Schulich School of Medicine & Dentistry, Western University, London ON, Canada.
authors.forename: Julia
authors.initials: J
authors.lastname: Poirier
authors.affiliation: 1 Department of Epidemiology & Biostatistics, Schulich School of Medicine & Dentistry, Western University, London ON, Canada.|2 Robarts Clinical Trials of Robarts Research Institute, Western University, London ON, Canada.
authors.forename: G Y
authors.initials: GY
authors.lastname: Zou
authors.affiliation: 1 Department of Epidemiology & Biostatistics, Schulich School of Medicine & Dentistry, Western University, London ON, Canada.
authors.forename: John
authors.initials: J
authors.lastname: Koval
mesh_terms: D056808:Biostatistics / Q000379:methods; D016000:Cluster Analysis; D017714:Community-Acquired Infections / Q000628:therapy; D003198:Computer Simulation; D016001:Confidence Intervals; D006801:Humans; D007902:Length of Stay; D015233:Models, Statistical; D011014:Pneumonia / Q000628:therapy; D016032:Randomized Controlled Trials as Topic / Q000379:methods* / Q000706:statistics & numerical data
keywords: asymmetry; cluster randomization trial; generalized confidence interval; lognormal mean; method of variance estimates recovery