title: An introduction to meta-analysis within the framework of multilevel modelling using the probability of success of root canal treatment as an illustration.
abstract: OBJECTIVE
  To introduce the statistical methodology of meta-analysis within the framework of multilevel modelling (MLM) using an illustrative example.
  
  
  METHODS
  In meta-analysis it is important that the quantitative pooling of study results should be carried out in conjunction with careful consideration of the variation apparent between studies. If statistical heterogeneity is found to be significant, it is due, at least in part, to clinical heterogeneity. It is possible to account for clinical heterogeneity by including covariates that are thought to be responsible, using meta-regression.
  
  
  METHODS
  A total of 38 studies of root canal treatment outcome were identified as being suitable for introducing the meta-analysis methodology. Two covariates were considered for modelling: a 'loose' or 'strict' (loose--incomplete radiographic healing; strict--complete radiographic healing) criterion for judging outcome of treatment and the year in which the study was performed.
  
  
  RESULTS
  There was considerable statistical heterogeneity between the study results. The effect of employing loose criteria for judging success significantly increased the probability of success when compared to employing strict criteria. Furthermore, the variance between studies was significantly reduced when this covariate was included in the modelling process when compared to the variation estimated in the model which did not consider covariates.
  
  
  CONCLUSIONS
  MLM is a good facilitator for meta-analysis and meta-regression.
authors.affiliation: Biostatistics Unit, Eastman Dental Institute for Oral Health Care Sciences, University College London, UK.
authors.forename: J D
authors.initials: JD
authors.lastname: Lewsey
authors.forename: M S
authors.initials: MS
authors.lastname: Gilthorpe
authors.forename: K
authors.initials: K
authors.lastname: Gulabivala
mesh_terms: D000704:Analysis of Variance; D003788:Dental Pulp Diseases / Q000000981:diagnostic imaging / Q000628:therapy; D006801:Humans; D016015:Logistic Models; D015201:Meta-Analysis as Topic; D015233:Models, Statistical; D010483:Periapical Diseases / Q000000981:diagnostic imaging / Q000628:therapy; D011336:Probability; D011446:Prospective Studies; D011859:Radiography; D012044:Regression Analysis; D012189:Retrospective Studies; D012390:Root Canal Therapy / Q000706:statistics & numerical data*; D016896:Treatment Outcome; D014945:Wound Healing