title: Methodological quality and reporting of generalized linear mixed models in clinical medicine (2000-2012): a systematic review.
abstract: BACKGROUND
  Modeling count and binary data collected in hierarchical designs have increased the use of Generalized Linear Mixed Models (GLMMs) in medicine. This article presents a systematic review of the application and quality of results and information reported from GLMMs in the field of clinical medicine.
  
  
  METHODS
  A search using the Web of Science database was performed for published original articles in medical journals from 2000 to 2012. The search strategy included the topic "generalized linear mixed models","hierarchical generalized linear models", "multilevel generalized linear model" and as a research domain we refined by science technology. Papers reporting methodological considerations without application, and those that were not involved in clinical medicine or written in English were excluded.
  
  
  RESULTS
  A total of 443 articles were detected, with an increase over time in the number of articles. In total, 108 articles fit the inclusion criteria. Of these, 54.6% were declared to be longitudinal studies, whereas 58.3% and 26.9% were defined as repeated measurements and multilevel design, respectively. Twenty-two articles belonged to environmental and occupational public health, 10 articles to clinical neurology, 8 to oncology, and 7 to infectious diseases and pediatrics. The distribution of the response variable was reported in 88% of the articles, predominantly Binomial (n = 64) or Poisson (n = 22). Most of the useful information about GLMMs was not reported in most cases. Variance estimates of random effects were described in only 8 articles (9.2%). The model validation, the method of covariate selection and the method of goodness of fit were only reported in 8.0%, 36.8% and 14.9% of the articles, respectively.
  
  
  CONCLUSIONS
  During recent years, the use of GLMMs in medical literature has increased to take into account the correlation of data when modeling qualitative data or counts. According to the current recommendations, the quality of reporting has room for improvement regarding the characteristics of the analysis, estimation method, validation, and selection of the model.
authors.affiliation: CIBER de Epidemiología y Salud Pública (CIBERESP), Barcelona, Spain; Bioestadística, Departament de Salut Pública, Universitat de Barcelona, Barcelona, Spain; Departament de Ciencies Basiques, Universitat Internacional de Catalunya, Barcelona, Spain; Servei d'Epidemiologia, Agència de Salut Pública de Barcelona, Barcelona, Spain.
authors.forename: Martí
authors.initials: M
authors.lastname: Casals
authors.affiliation: Departament de Fisioteràpia (unitat de Bioestadística), Universitat Internacional de Catalunya, Barcelona, Spain.
authors.forename: Montserrat
authors.initials: M
authors.lastname: Girabent-Farrés
authors.affiliation: Bioestadística, Departament de Salut Pública, Universitat de Barcelona, Barcelona, Spain.
authors.forename: Josep L
authors.initials: JL
authors.lastname: Carrasco
mesh_terms: D015510:Clinical Medicine / Q000706:statistics & numerical data*; D016014:Linear Models