title: Systematic review finds overlapping reviews were not mentioned in every other overview.
abstract: OBJECTIVE
  The objective of this study was to determine if the authors mention overlapping reviews in overviews (reviews of reviews). In addition, we aimed to calculate the actual overlap in published overviews using newly introduced, validated measures.
  
  
  METHODS
  We systematically searched for overviews from 2009 to 2011. Reviews included in the overviews were obtained. Tables (review×primary publication) were generated for each overview. The first occurrence of a primary publication is defined as the index publication. We calculated the "corrected covered area" (CCA) as a measure of overlap by dividing the frequency of repeated occurrences of the index publication in other reviews by the product of index publications and reviews, reduced by the number of index publications. Subgroup analyses were performed to investigate further differences in the overviews.
  
  
  RESULTS
  Only 32 of 60 overviews mentioned overlaps. The median CCA was 4.0. Validation of the CCA and other overlap measures was in accordance with our predefined hypotheses. The degree of overlap tended to be higher in health technology assessment reports than in journal publications and was higher with increasing numbers of publications.
  
  
  CONCLUSIONS
  Overlaps must be reported in well-conducted overviews, and this can comprehensively be accomplished using the CCA method.
authors.affiliation: Institute for Research in Operative Medicine (IFOM), Witten/Herdecke University, Ostmerheimer Str. 200, Building 38, D-51109, Cologne, Germany. Electronic address: dawid.pieper@uni-wh.de.
authors.forename: Dawid
authors.initials: D
authors.lastname: Pieper
authors.affiliation: Institute for Research in Operative Medicine (IFOM), Witten/Herdecke University, Ostmerheimer Str. 200, Building 38, D-51109, Cologne, Germany.
authors.forename: Sunya-Lee
authors.initials: SL
authors.lastname: Antoine
authors.affiliation: Institute for Research in Operative Medicine (IFOM), Witten/Herdecke University, Ostmerheimer Str. 200, Building 38, D-51109, Cologne, Germany.
authors.forename: Tim
authors.initials: T
authors.lastname: Mathes
authors.affiliation: Institute for Research in Operative Medicine (IFOM), Witten/Herdecke University, Ostmerheimer Str. 200, Building 38, D-51109, Cologne, Germany.
authors.forename: Edmund A M
authors.initials: EA
authors.lastname: Neugebauer
authors.affiliation: Institute for Research in Operative Medicine (IFOM), Witten/Herdecke University, Ostmerheimer Str. 200, Building 38, D-51109, Cologne, Germany.
authors.forename: Michaela
authors.initials: M
authors.lastname: Eikermann
mesh_terms: D019317:Evidence-Based Medicine; D006801:Humans; D016032:Randomized Controlled Trials as Topic; D012107:Research Design; D012196:Review Literature as Topic
keywords: Evidence-based medicine; Information science; Meta-analysis; Methods; Research design; Systematic review