title: Are clustering effects accounted for in statistical analysis in leading dental specialty journals?
abstract: OBJECTIVE
  In dental research multiple site observations within patients or taken at various time intervals are commonplace. These clustered observations are not independent; statistical analysis should be amended accordingly. This study aimed to assess whether adjustment for clustering effects during statistical analysis was undertaken in five specialty dental journals.
  
  
  METHODS
  Thirty recent consecutive issues of Orthodontics (OJ), Periodontology (PJ), Endodontology (EJ), Maxillofacial (MJ) and Paediatric Dentristry (PDJ) journals were hand searched. Articles requiring adjustment accounting for clustering effects were identified and statistical techniques used were scrutinized.
  
  
  RESULTS
  Of 559 studies considered to have inherent clustering effects, adjustment for this was made in the statistical analysis in 223 (39.1%). Studies published in the Periodontology specialty accounted for clustering effects in the statistical analysis more often than articles published in other journals (OJ vs. PJ: OR=0.21, 95% CI: 0.12, 0.37, p<0.001; MJ vs. PJ: OR=0.02, 95% CI: 0.00, 0.07, p<0.001; PDJ vs. PJ: OR=0.14, 95% CI: 0.07, 0.28, p<0.001; EJ vs. PJ: OR=0.11, 95% CI: 0.06, 0.22, p<0.001). A positive correlation was found between increasing prevalence of clustering effects in individual specialty journals and correct statistical handling of clustering (r=0.89).
  
  
  CONCLUSIONS
  The majority of studies in 5 dental specialty journals (60.9%) examined failed to account for clustering effects in statistical analysis where indicated, raising the possibility of inappropriate decreases in p-values and the risk of inappropriate inferences.
authors.affiliation: Barts and The London School of Medicine and Dentistry, Institute of Dentistry, Queen Mary University of London, United Kingdom. padhraig.fleming@gmail.com
authors.forename: Padhraig S
authors.initials: PS
authors.lastname: Fleming
authors.forename: Despina
authors.initials: D
authors.lastname: Koletsi
authors.forename: Argy
authors.initials: A
authors.lastname: Polychronopoulou
authors.forename: Theodore
authors.initials: T
authors.lastname: Eliades
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Pandis
mesh_terms: D016000:Cluster Analysis; D003627:Data Interpretation, Statistical; D018865:Dental Research; D006801:Humans; D007598:Journalism, Dental; D010506:Periodicals as Topic; D012107:Research Design