Relationship between category size and journals' impact factor: implications for emergency medicine journals and researchers.

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.

Materials And 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.

Conclusion: 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.