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It’s pretty likely you’ll have heard of impact factors, either through colleagues talking about them in the lab, or from a journal homepage advertising its latest score. Whilst impact factor is a relatively artificial value, it is something that journal editors, scientists and some funding agencies take seriously. It’s therefore important to understand what it measures, and what it means to you.
Impact factor is a fairly crude way of assessing the scientific impact of papers published in a particular journal. In this case, impact is defined as the number of citations a typical paper will have over a proscribed period. The most commonly-quoted impact factor data are compiled by the Institute for Scientific Information (ISI). The ISI impact factor of a journal for a given year is defined as the average number of times that papers published in that journal over the previous two years were cited in the given year.
The 2012 impact factor for a given journal will therefore be:
Other measures of impact exist, and they may measure citations over a longer period of time. However, when someone refers to “impact factor”, they are generally referring to the ISI measure.
Unlike h-factor (discussed by Nick here), impact factor is not a measure of an individual scientist’s success. Nor is it necessarily an indicator of how “good” a paper is. Just because a paper is published in a journal with a relatively low impact factor does not mean it will not be widely read (and cited). It certainly does not mean that the data it contains are any less valid. Likewise, the impact factor of journals with high impact factors can be skewed by relatively few very highly cited papers. An “average” paper in one of these journals is therefore unlikely to be as widely cited as the journal impact factor suggests.
Crucially, impact factor takes the number of citations as the key measure of scientific impact. However, number of citations may not always be the best indicator of impact. For example, papers in different subject areas may expect to receive different numbers of citations. An incremental advance in a large, rapidly moving field may attract a large number of citations; whereas a comprehensive and detailed study which hugely advances a relatively niche field may not.
Journals (particularly ones which score highly on the impact factor scale) take impact factors very seriously. A journal with a high impact factor will therefore generally seek to preserve this with stricter reviewing and editorial policies. It is important to consider this when you are deciding which journal to send your treasured research article to. Whilst you may think your work is the most interesting thing you could spend all your waking hours on, the editors and reviewers may not agree, and the higher impact factor-scored journals are more likely to reject your manuscript. However, higher-rated journals are likely to have a higher readership, and more people may be made aware of your work.
A paper in a high-impact journal will undoubtedly look good on your CV. However, the most important thing to think about when submitting your toil to the mercies of journal editors is who you want your paper to be read by. As long as the right people get to read your paper, it will influence how they think about your subject, and therefore have an “impact”. Rather than using impact factor as a major guideline, think about which journals you read most often, as their readership is the one you most want to attract. After all, the most important thing about a paper is who reads it, not where they read it.
What do you think: is impact factor important to you?