When are readership counts as useful as citation counts? Scopus versus Mendeley for LIS journals

In theory, articles can attract readers in the social reference sharing site Mendeley before they can attract citations and so Mendeley altmetrics could provide early indications of article impact. This article investigates the influence of time on the number of Mendeley readers of an article through a theoretical discussion and an investigation into the relationship between counts of readers of, and citations to, four general Library and Information Science (LIS) journals. For this discipline, it takes about seven years for articles to attract as many Scopus citations as Mendeley readers and after this the Spearman correlation between readers and citers is stable at about 0.6 for all years. This suggests that Mendeley readership counts may be useful impact indicators for both newer and older articles. The lack of dates for individual Mendeley article readers and an unknown bias towards more recent articles mean that readership data should be normalised individually by year, however, before making any comparisons between articles published in different years.