title: Multilevel survival analysis of amalgam restorations amongst RAF personnel.
abstract: OBJECTIVE
  To introduce the concepts of multilevel survival analysis through an investigation into the longevity of amalgam restorations.
  
  
  METHODS
  The multilevel Cox proportional hazard model is illustrated using amalgam restoration data comprising three levels: repeated restorations at level-1, teeth at level-2, and subjects at level-3. The outcome was duration of amalgam restoration survival. Single-level and multilevel Cox methods are contrasted.
  
  
  METHODS
  The data were from a survey of amalgam restorations (reported elsewhere), involving 200 RAF personnel aged between 16 and 37 years at enlistment between 1947 and 1979, having served continuously for a minimum of 16 years prior to 1994.
  
  
  RESULTS
  Differences existed between single-level and multilevel methods; the latter being the method of choice. Initial caries experience was a good predictor of longevity. Molar teeth fared worse than pre-molars and MOD & B, MOD & L, and MOD & BL restorations experienced considerably greater risk of failure than did MOD, MO, DO and MO/DO ext types, which in turn fared worse than occlusal restorations. Root-treated and pinned teeth also experienced an elevated risk of premature failure. There was a moderate but significant increase in restoration failure amongst subjects who were seen by more dentists throughout their service.
  
  
  CONCLUSIONS
  The application of multilevel modelling to survival analysis provides an appropriate and powerful solution to the problem of lack of independence amongst dental restorations. It is beneficial that studies undertake a multilevel analysis in preference to ignoring hierarchy or omitting swathes of information in order to perform a single-level analysis.
authors.affiliation: Biostatistics Unit, Eastman Dental Institute for Oral Health Care Sciences, University College London, UK. m.gilthorpe@eastman.ucl.ac.uk
authors.forename: Mark S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: Martin T
authors.initials: MT
authors.lastname: Mayhew
authors.forename: John S
authors.initials: JS
authors.lastname: Bulman
mesh_terms: D000293:Adolescent; D000328:Adult; D004245:DMF Index; D003723:Dental Amalgam; D019232:Dental Restoration Failure; D003793:Dental Restoration, Permanent / Q000706:statistics & numerical data*; D006801:Humans; D016014:Linear Models; D008889:Military Personnel; D015233:Models, Statistical; D016016:Proportional Hazards Models; D012307:Risk Factors; D016019:Survival Analysis; D006113:United Kingdom
chemical_list: D003723:Dental Amalgam