Garfield grounds his warning against individual-level use of the journal impact factor in one recurring mechanism: wide citation variance among articles within the same journal

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Garfield's warning against using the journal impact factor to judge individuals is not a bare assertion — across at least three of his own primary texts spanning 1996 to 2001, he gives the same specific technical reason for it. His 1998 letter to Der Unfallchirurg attributes the underlying empirical case to another researcher by name: "There is wide variation from article to article within a single journal as has been widely documented by Per O. Seglen of Norway and others." His 2001 Nature letter states the positive counterpart directly: it "would be more relevant to use the actual impact (citation frequency) of individual papers in evaluating the work of individual scientists rather than using the journal impact factor as a surrogate," calling the surrogate practice "fraught with difficulties, as Seglen and others have pointed out." His earlier 1996 BMJ essay frames the same substitution as a category error: "Using the journal's average citation impact instead of the actual article impact is tantamount to grading by the prestige of the journal involved."

The mechanism is arithmetic, not merely institutional caution: a journal's impact factor is a mean across all its articles, and if citation counts vary widely from article to article within that journal — the empirical finding Per O. Seglen documented — then the mean cannot stand in for any one article's, or any one author's, actual citation count. This gives Garfield's warning (see the companion note) a stated evidentiary basis rather than leaving it as an inventor's unexplained discomfort with his own metric's misuse.