title: Introduction to multilevel modelling in dental research.
abstract: OBJECTIVE
  To explain the concepts and application of multilevel modelling (MLM) and how it can be applied to the analysis of dental research data.
  
  
  METHODS
  Methodological in nature, this article introduces MLM through actual and hypothetical dental examples.
  
  
  METHODS
  Examples of MLM in periodontal research illustrate cross-sectional and longitudinal analyses of hierarchical dental data. Multilevel multivariate modelling is illustrated by an example in orthodontic research. The potential implications of study design in the context MLM is extensively discussed, including study sample size and accounting for examiner variability.
  
  
  RESULTS
  The fine detail and greater insight provided by MLM enables dental researchers to review and revise old hypotheses and to generate new ones that could not be addressed by single-level methods.
  
  
  CONCLUSIONS
  MLM has the potential to increase our understanding of oral disease and health and offers the opportunity to rethink some aspects of dental research procedure.
authors.affiliation: Biostatistics Unit,Eastman Dental Institute for Oral Health Care Sciences, University College London, UK. m.gilthorpe@eastman.ucl.ac.uk
authors.forename: M S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: I H
authors.initials: IH
authors.lastname: Maddick
authors.forename: A
authors.initials: A
authors.lastname: Petrie
mesh_terms: D000704:Analysis of Variance; D003430:Cross-Sectional Studies; D003625:Data Collection; D003627:Data Interpretation, Statistical; D018865:Dental Research / Q000379:methods*; D006801:Humans; D008137:Longitudinal Studies; D015233:Models, Statistical; D015999:Multivariate Analysis; D015588:Observer Variation; D009970:Orthodontics / Q000379:methods; D010517:Periodontics / Q000379:methods; D015203:Reproducibility of Results; D012107:Research Design; D018401:Sample Size