title: The application of multilevel modelling to periodontal research data.
abstract: OBJECTIVE
  To explain the theory of multilevel modelling and demonstrate its application in the analysis of dental research data.
  
  
  METHODS
  Multilevel modelling was introduced using dental data comprising four levels: repeated measurements at level-1, sites at level-2, teeth at level-3, and subjects at level-4. Variance components models (which have no explanatory variables) were evaluated for all outcome measures. Explanatory variables were added to the models with outcomes for both lifetime cumulative attachment loss and pocket probing depth. Salient features of the multilevel models were discussed.
  
  
  METHODS
  Research data were obtained from a longitudinal survey of periodontal disease conducted on 100 white male trainee engineers aged between 16 and 20 years entering the apprentice training school at Royal Air Force Halton, England.
  
  
  RESULTS
  The statistical methods revealed that periodontal measures demonstrate considerable variation at all levels of the multilevel structure. Models for lifetime cumulative attachment loss and pocket probing depth illustrated that risk factors operated at more than one level. Supragingival calculus was a risk factor at the subject-level (subjects experiencing more sites with the condition had greater attachment loss and greater pocketing) whilst there was apparently a protective effect occurring at the site (sites with the condition had less attachment loss and less pocketing).
  
  
  CONCLUSIONS
  This study demonstrates that multilevel modelling is a more powerful research tool than single-level techniques for the analysis of hierarchical dental data. Researchers using these techniques are well equipped to analyse complex hierarchical data structures, such as those often found within dentistry.
authors.affiliation: Biostatistics Unit, Eastman Dental Institute for Oral Health Care Sciences, University College London, United Kingdom. m.gilthorpe@eastman.ucl.ac.uk
authors.forename: M S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: G S
authors.initials: GS
authors.lastname: Griffiths
authors.forename: I H
authors.initials: IH
authors.lastname: Maddick
authors.forename: A T
authors.initials: AT
authors.lastname: Zamzuri
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D003627:Data Interpretation, Statistical; D003728:Dental Calculus / Q000453:epidemiology; D003773:Dental Plaque / Q000453:epidemiology; D018865:Dental Research / Q000379:methods*; D004739:England / Q000453:epidemiology; D006801:Humans; D008297:Male; D008889:Military Personnel; D015233:Models, Statistical; D010512:Periodontal Index; D010517:Periodontics / Q000379:methods*