Obsolescence of the literature: A study of included studies in Cochrane reviews

Ageing or obsolescence describes the process of declining use of a particular publication over time and can affect the results of a citation analyses as the length of citation window can change rankings. Obsolescence may not only vary across fields but also across subfields or sub-disciplines. The aim of this study is to determine the sub-disciplinary differences of obsolescence on a larger scale allowing for differences over time as well. The study presents the results of an analysis of 82,759 references across 53 healthcare and health policy topics. The references in this study are extracted from systematic reviews published from 2012 to 2016. The analyses of obsolescence include median citation age and mean citation age. This study finds that the median citation age and the mean citation age differ considerably across groups. For the latter indicator, an analysis of the confidence intervals confirms these differences. Using the subfield categorisation from Cochrane review groups, we found larger differences across subfields than in the citing half-lives published by Journal Citation Reports. Obsolescence is important to consider when setting the length of the citation windows. This study emphasises the vast differences across health sciences subfields. The length of the citation period is thus highly important for the results of a bibliometric evaluation or study covering fields with very varying obsolescence rates.