Delayed open access: An overlooked high-impact category of openly available scientific literature

Delayed open access (OA) refers to scholarly articles in subscription journals made available openly on the web directly through the publisher at the expiry of a set embargo period. Although a substantial number of journals have practiced delayed OA since they started publishing e-versions, empirical studies concerning OA have often overlooked this body of literature. This study provides comprehensive quantitative measurements by identifying delayed OA journals and collecting data concerning their publication volumes, embargo lengths, and citation rates. Altogether, 492 journals were identified, publishing a combined total of 111,312 articles in 2011; 77.8% of these articles were made OA within 12 months from publication, with 85.4% becoming available within 24 months. A journal impact factor analysis revealed that delayed OA journals have citation rates on average twice as high as those of closed subscription journals and three times as high as immediate OA journals. Overall, the results demonstrate that delayed OA journals constitute an important segment of the openly available scholarly journal literature, both by their sheer article volume and by including a substantial proportion of highimpact journals.