title: Random-effects models in investigating the effect of vitamin A in childhood diarrhea.
abstract: OBJECTIVE
  By adopting more appropriate and powerful statistical methods that fully exploit longitudinal structure, we re-analyze and extend previously published results from a large community trial to investigate the effect of vitamin A supplementation on the prevalence and severity of diarrhea in young children.
  
  
  METHODS
  Generalized linear mixed models were used to allow for repeated measures in a reanalysis of a double-blind, randomized, placebo-controlled community trial conducted in a cohort of children in northeastern Brazil during 1 year. The response variable was weekly number of days with diarrhea for each child, and Markov Chain Monte Carlo methods were used to estimate model parameters.
  
  
  CONCLUSIONS
  Random effects suitably accounted for the underlying heterogeneity between and within children, and our longitudinal analysis shows a significant beneficial effect of vitamin A supplementation that was inconclusive in previously reported simple summary analyses of these data. Risk for diarrhea infection was estimated to be 1.57 times greater for a child administered a placebo as opposed to vitamin A (95% credible interval, 1.17-2.12). Additionally, we identified previously unreported temporal effects in these data, showing a decreasing daily probability of diarrhea for both groups during the trial and treatment-time interaction.
authors.affiliation: National School of Public Health, Oswaldo Cruz Foundation, Rio de Janeiro, Brazil. valeska@fiocruz.br
authors.forename: Valeska L
authors.initials: VL
authors.lastname: Andreozzi
authors.forename: Trevor C
authors.initials: TC
authors.lastname: Bailey
authors.forename: Flávio F
authors.initials: FF
authors.lastname: Nobre
authors.forename: Claudio J
authors.initials: CJ
authors.lastname: Struchiner
authors.forename: Maurício L
authors.initials: ML
authors.lastname: Barreto
authors.forename: Ana M O
authors.initials: AM
authors.lastname: Assis
authors.forename: Leonor M P
authors.initials: LM
authors.lastname: Santos
mesh_terms: D001499:Bayes Theorem; D001938:Brazil / Q000453:epidemiology; D002675:Child, Preschool; D003627:Data Interpretation, Statistical; D003968:Diarrhea, Infantile / Q000453:epidemiology / Q000517:prevention & control*; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D007223:Infant; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D013997:Time Factors; D014801:Vitamin A / Q000008:administration & dosage*
chemical_list: D014801:Vitamin A