title: Inference of nested variance components in a longitudinal myopia intervention trial.
abstract: This paper was motivated by a double-blind randomized clinical trial of myopia intervention. In addition to the primary goal of comparing treatment effects, we are concerned with the modelling of correlation that may come from two possible sources, one among the longitudinal observations and the other between measurements taken from both eyes per subject. The data are nested repeated measurements. We suggest three models for analysis. Each one expresses the correlation differently in various covariance structures. We articulate their differences and describe the implementations in estimation using commercial statistical software. The computer output can be further utilized to perform model selection with Schwarz criterion. Simulation studies are conducted to evaluate the performance under each model. Data of the myopia intervention trial are reanalysed with these models for illustration. The results indicate that atropine is more effective in reducing the progression rate, the rates are homogeneous across subjects, and, among the suggested models, the one with independent random effects of two eyes fits best. We conclude that model selection is a crucial step before making inference with estimates; otherwise the correlation may be attributed incorrectly to a different mechanism. The same conclusion applies to other variance components as well.
authors.affiliation: Division of Biostatistics, Institute of Epidemiology, National Taiwan University, Taipei 100, Taiwan. ckhsiao@ha.mc.ntu.edu.tw
authors.forename: Chuhsing Kate
authors.initials: CK
authors.lastname: Hsiao
authors.forename: Miao-Yu
authors.initials: MY
authors.lastname: Tsai
authors.forename: Ho-Min
authors.initials: HM
authors.lastname: Chen
mesh_terms: D000704:Analysis of Variance; D001285:Atropine / Q000627:therapeutic use; D002648:Child; D003198:Computer Simulation; D004311:Double-Blind Method; D005139:Eyeglasses / Q000592:standards; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D015233:Models, Statistical; D009184:Mydriatics / Q000627:therapeutic use; D047728:Myopia, Degenerative / Q000628:therapy*; D016032:Randomized Controlled Trials as Topic / Q000379:methods*
chemical_list: D009184:Mydriatics; D001285:Atropine