These measures fall into two broad categories:
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unweighted — all citations are given the same consideration
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weighted — some citations are considered worth more than others
The best known measure of impact is the Journal Impact Factor developed by Eugene Garfield and measured using the Journal Citation Reports database. Other tools have since been developed in order to address some of the limits of this particular method. With a new shift toward article level metrics, there is growing debate as to the appropriateness of journal impact factors in promotion and tenure decisions.