title: The application of multilevel modeling in the analysis of longitudinal periodontal data--part I: absolute levels of disease.
abstract: BACKGROUND
  Statistical analyses of periodontal data that average site measurements to subject mean values are unable to explore the site-specific nature of periodontal diseases. Multilevel modeling (MLM) overcomes this, taking hierarchical structure into account. MLM was used to investigate longitudinal relationships between the outcomes of lifetime cumulative attachment loss (LCAL) and probing depth (PD) in relation to potential risk factors for periodontal disease progression.
  
  
  METHODS
  One hundred males (mean age 17 years) received a comprehensive periodontal examination at baseline and at 12 and 30 months. The resulting data were analyzed in two stages. In stage one (reported here), the absolute levels of disease were analyzed in relation to potential risk factors; in stage two (reported in a second paper), changes in disease patterns over time were analyzed in relation to the same risk factors. Each approach yielded substantially different insights.
  
  
  RESULTS
  For absolute levels of disease, subject-level risk factors (covariates) had limited prediction for LCAL/PD throughout the 30-month observation period. Tooth position demonstrated a near linear relationship for both outcomes, with disease increasing from anterior to posterior teeth. Sites with subgingival calculus and bleeding on probing demonstrated more LCAL and PD, and supragingival calculus had an apparently protective effect. Covariates had more "explanatory power" for the variation in PD than for the variation in LCAL, suggesting that LCAL and PD might be generally associated with a different profile of covariates.
  
  
  CONCLUSIONS
  This study provides, for a relatively young cohort, considerable insights into the factors associated with early-life periodontal disease and its progression at all levels of the natural hierarchy of sites within teeth within subjects.
authors.affiliation: Department of Periodontology, Leeds Dental Institute, University of Leeds, Leeds, UK.
authors.forename: Yu-Kang
authors.initials: YK
authors.lastname: Tu
authors.forename: Mark S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: Gareth S
authors.initials: GS
authors.lastname: Griffiths
authors.forename: Ian H
authors.initials: IH
authors.lastname: Maddick
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Eaton
authors.forename: Newell W
authors.initials: NW
authors.lastname: Johnson
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D015986:Confounding Factors, Epidemiologic; D003627:Data Interpretation, Statistical; D018865:Dental Research / Q000379:methods*; D018450:Disease Progression; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008889:Military Personnel; D015233:Models, Statistical; D010510:Periodontal Diseases / Q000453:epidemiology*; D012307:Risk Factors; D006113:United Kingdom / Q000453:epidemiology