title: Ratio variables in regression analysis can give rise to spurious results: illustration from two studies in periodontology.
abstract: OBJECTIVE
  For over a century, statisticians have highlighted concerns about the inappropriate use of ratio variables in correlation and regression analysis. However, little attention has been paid to these concerns in medical and dental research. The use of ratio variables in correlation and regression analysis can give rise to spurious results due to inappropriate model specification and mathematical coupling, leading to serious misinterpretation of data and consequently to incorrect study conclusions.
  
  
  METHODS
  Data were reanalysed from two recently published articles: one on the efficacy of guided tissue regeneration on root coverage; the other a randomised controlled trial comparing three surgical approaches in the treatment of periodontal infrabony defects. The reanalysis was performed to examine whether the assumptions behind the correlation/regression analyses have been seriously violated in these two studies, and to see if the interpretation of results is tenable.
  
  
  RESULTS
  Use of ratio variables seriously violated the assumptions underpinning the statistical methods utilised in these two studies, and consequently the conclusions were substantially misleading. Recommendations made in these studies were not tenable.
  
  
  CONCLUSIONS
  The reanalyses illustrate how the inappropriate use of ratio variables remains prevalent in dental research, leading to incorrect interpretation of the evidence. This emphasises the need for collaboration between clinicians and statisticians to avoid the risk of yielding erroneous conclusions from flawed statistical analyses.
authors.affiliation: Biostatistics Unit, School of Medicine, University of Leeds, 24 Hyde Terrace, LS2 9LN, Leeds, UK.
authors.forename: Yu-Kang
authors.initials: YK
authors.lastname: Tu
authors.forename: Valerie
authors.initials: V
authors.lastname: Clerehugh
authors.forename: Mark S
authors.initials: MS
authors.lastname: Gilthorpe
mesh_terms: D016301:Alveolar Bone Loss / Q000601:surgery*; D000704:Analysis of Variance; D003627:Data Interpretation, Statistical; D018865:Dental Research / Q000379:methods*; D005889:Gingival Recession / Q000601:surgery*; D016557:Guided Tissue Regeneration, Periodontal; D006801:Humans; D008433:Mathematics; D017063:Outcome Assessment, Health Care / Q000379:methods*; D012044:Regression Analysis; D012107:Research Design