Are classic references cited first? An analysis of citation order within article sections

Early citations within an article section may have an agenda-setting role but contribute little to the new research. To investigate whether this practice may be common, this article assesses whether the average impact of cited references is influenced by the order in which they are cited within article sections. This is tested on 1,683,299,868 citations to 41,068,375 unique journal articles from 1,470,209 research articles in the PubMed Open Access collection, split into 22 fields. The results show that the first cited article in the Introduction and Background have much higher average citation impacts than later articles, and the same is true to a lesser extent for the Discussion and Conclusion in most fields, but not the Methods and Results. The findings do not prove that early citations are less central to the citing article but nevertheless add to previous evidence suggesting that this practice may be widespread. It may therefore be useful to distinguish between initial introductory citations when evaluating citation impact, or to use impact indicators that implicitly or explicitly give less weight to the citation counts of highly cited articles.