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Evaluating scientific quality is a notoriously difficult problem which has no standard solution.Ideally, published scientific results should be scrutinised by true experts in the field and given scores for quality and quantity according to established rules.In practice, however, what is called peer review is usually performed by committees with general competence rather than with the specialist's insight that is needed to assess primary research data.Committees tend, therefore, to resort to secondary criteria like crude publication counts, journal prestige, the reputation of authors and institutions, and estimated importance and relevance of the research field, 1 making peer review as much of a lottery as of a rational process. 2 3 On this background, it is hardly surprising that alternative methods for evaluating research are being sought, such as citation rates and journal impact factors, which seem to be quantitative and objective indicators directly related to published science.The citation data are obtained from a database produced by the Institute for Scientific Information (ISI) in Philadelphia, which continuously records scientific citations as represented by the reference lists of articles from a large number of the world's scientific journals.The references are rearranged in the database to show how many times each publication has been cited within a certain period, and by whom, and the results are published as the Science Citation Index (SCI).On the Is the journal impact factor really representative of the individual journal articles? Relation of journal impact factor and citation rate of articleFor the journal's impact factor to be reasonably representative of its articles, the citation rate of individual articles in the journal should show a narrow distribution, preferably a Gaussian distribution, around Summary points• Use of journal impact factors conceals the difference in article citation rates (articles in the most cited half of articles in a journal are cited 10 times as often as the least cited half) • Journals' impact factors are determined by technicalities unrelated to the scientific quality of their articles • Journal impact factors depend on the research field: high impact factors are likely in journals covering large areas of basic research with a rapidly expanding but short lived literature that use many references per article