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The heterogeneity of author self‐citations is highlighted and a systematic scheme for their classification is presented. Self‐citations are either synchronous or diachronous and each of these classes or genera has four subclasses or species. The distribution of self‐citations among the four species is governed by a number of factors including collaborative tendencies in the discipline or research specialty and the relative statuses of the collaborating authors. The classification of self‐citations may be applied to study aspects of research collaboration and the matter of egotism in scholarly work.

This article reviews the principal correlational studies employing citation counts as criterion measures for assessing the impact of scientific scholarship. The rationale and limitations of such measures and studies are discussed. New evidence on the validity of citation criteria is presented based on a sample of 870 cancer research papers, divided into three groups ("first-order" papers, abstracted in the Year Book of Cancer; "secondorder" papers, listed but not abstracted in the yearbook; and "average-order" papers, a representative cross section of research papers unlisted in the yearbook). Results consistently show that highly rated papers are more highly cited over the ensuing five years after publication, or when controls are introduced for selfcitations, for the influence of listing in the yearbook, and for language and country of authorship. The implicatlons of results are discussed.

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Analyses of a sample of cancer research papers previously judged to be of high quality and of a random sample from the world literature, indicate that nineteen countries are the key producers of cancer research literature. Quality papers were much less dispersed among countries than ordinary papers and there was a high and statistically significant correlation be tween the quantity and the quality of productivity of countries.