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The impact factor for journal X = the number of citations in a given year to all articles published in journal X for the previous two years, divided by the number of articles in that journal for those previous two years
What should concern you about cited reference searching and impact factors?
Citation rates vary widely from field to field and shouldn't be taken at face value, but considered relative to the field of research
Citation rate may be based on a few prolific authors citing each other, including self citations
Citation searching works better for journal articles than books
Coverage of your particular field in the citation database may be weak
Cross-disciplinary research may produce fewer citations
Distribution of the citations over time might be more indicative of their importance than an initial high citation count shortly after publication
Impact factors are a measure the journal as a whole, not individual papers or researchers
Impact factors may play a too important role in hiring and tenure decisions without considering other measures
Some journals also cite articles in editorials, reviews, news and other non-research articles to increase the number of cites
The research may too recent and not widely known, like emerging fields
The quality of the journal producing the citation
There is a growing tendency of some researchers to go after topics likely to get into high-impact journals, which jeopardizes creativity, can skew the course or even slow the pace of science
There is no guarantee that every paper which ought to be cited will be cited. An un-cited author may be ahead of his peers. Mendel and his genetics work went unappreciated for years