Highly cited old papers and the reasons why they continue to be cited

This article describes a study to ascertain the reasons why certain old papers are still highly cited many years after their publication. Twenty-three old papers in the subject fields of physics and physical chemistry which are still highly cited were selected, and 978 of the papers that cited them in the period 1974–1975 were studied. A new typology of reasons for citing the papers was devised, and using this typology, it was found that about 40% of the citations were for historical reasons, but that in the remaining 60% of the cases, the old paper is still begin actively used. We discuss some discrepancies between our citation figures and citation figures quoted by Garfield. We also discuss a number of errors which were found, both in citing articles and in Science Citation Index. Finally, calculations indicate that there is a rule that each cited paper is referred to, on average, 1.05-1.15 times in every paper that cites it, and that this rule has general validity.