title: Relationship between category size and journals' impact factor: implications for emergency medicine journals and researchers.
abstract: OBJECTIVE
  We assessed the relationship between the size of the 39 Journal Citation Reports (JCR) medical categories and impact factor (IF) of journals in these categories, and the implications that it might have for emergency medicine (EM) journals.
  
  
  METHODS
  Using the 2010 JCR database, we calculated the mean IF, 5-year IF (5y-IF), Eigenfactor (EF), and Article Influence (AI) scores including all journals for each category. We also calculated a 'weighted IF' for all journals by dividing each journal IF by the mean IF of its category. We ranked EM journals according to IF and 'weighted IF' into all the journals included in the 39 categories. We assessed the relationship between category size and bibliometric scores by linear regression.
  
  
  RESULTS
  Category size varied from 252 journals (Pharmacology and Pharmacy) to 14 (Primary Healthcare), EM category occupying the 36th position (23 journals). The mean IF of EM category ranked in 34th position, 5-yIF in 32nd, EF in 34th, and AI in 34th position. Category size had a direct and significant association with mean IF, 5y-IF, and AI but not with mean EF. When the EM journals were ranked among all the journals according to their IF, only two (9%) were placed into the first quartile and raised up to eight (35%) when 'weighted IF' was considered.
  
  
  CONCLUSIONS
  There is a negative relationship between JCR size category and IF achieved by the journals. This places EM journals at a clear disadvantage because they represent one of the smallest clinical medical research disciplines.
authors.affiliation: aEditor-in-Chief, Emergencias, Department of Emergency Medicine, Hospital Clinic de Barcelona, Barcelona bAssociate Editor, Emergencias, Department of Emergency Medicine, Hospital Clínico San Carlos, School of Medicine, Universidad Complutense, Madrid, Spain cFormer Editor-in-Chief, Emergency Medicine, Department of Emergency Medicine, Royal Brisbane and Women's Hospital, School of Medicine, University of Queensland, Brisbane, Queensland, Australia dEditor-in-Chief, European Journal of Emergency Medicine, Accident and Emergency Medicine Academic Unit, The Chinese University of Hong Kong, Shatin, Hong Kong eEditor-in-Chief, Canadian Journal of Emergency Medicine, Department of Medicine, Division of Emergency Medicine, McMaster University, Hamilton, Ontario, Canada fEditor-in-Chief, Academic Emergency Medicine,Yale University School of Medicine, New Haven, Connecticut, USA.
authors.forename: Òscar
authors.initials: Ò
authors.lastname: Miró
authors.forename: Anthony F T
authors.initials: AF
authors.lastname: Brown
authors.forename: Colin A
authors.initials: CA
authors.lastname: Graham
authors.forename: James
authors.initials: J
authors.lastname: Ducharme
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Martin-Sanchez
authors.forename: David C
authors.initials: DC
authors.lastname: Cone
mesh_terms: D035843:Biomedical Research; D016208:Databases, Factual; D004635:Emergency Medicine; D006801:Humans; D055811:Journal Impact Factor; D010506:Periodicals as Topic; D011786:Quality Control