The journal impact factor is defined as all citations made in the past year from scholarly publications (journals, proceedings, and books) to a journal’s publications from the two years prior divided by the total number of publications (articles and reviews, i.e., citable items) published in the journal in those two prior years. So the 2020 journal impact factor uses citations made in 2020 to material from 2018 and 2019.
The h5-index used by Google Scholar is “the h-index for articles published in the last 5 complete years. It is the largest number h such that h articles published in 2015-2019 have at least h citations each.” For example, New Media & Society has an h5-index of 79, meaning that there are 79 papers with at least 79 citations (in this case, they range from 505 to 79 citations. Nature has an h5-index of 376 with the top paper, published in 2015, cited 27375 times).
For the field of communication science, I compiled a list with both metrics. I selected journals that I personally could imagine submitting to based on my research focus, so it is not intended to be exhaustive. I still included journals from political communication and journalism though, which are not my area of research.
The two metrics defined above obviously measure different things but they are (positively) related. Journals with high impact factors tend to also have high h5-indices. It’s perhaps instructive to look at “outliers”; I’ll pick the International Journal of Communication (low impact factor, high h5-index,) and Communication Monographs (high impact factor, low h5-index).
The International Journal of Communication’s Volume 13 contains 281 articles and 95 book reviews. Communication Monographs Volume 86 (2019) has 4 issues with 6 articles each, that’s 24 articles (two years worth go into the impact factor calculation as the denominator). That’s a huge difference, and despite both being “general” communication journals, they are just very different. Communication Monographs has an article on mediation analysis at the top of the list with 165 “eligible” citations; the 10th article has 6 citations. The International Journal of Communication’s top paper is on persuasion and has 17 citations; the 10th article has 7 citations.
Is the impact factor the better metric here because it adjusts for the volume? I’d say the International Journal of Communication has a high impact on the field in part precisely because of the high number of papers – there is more available potentially relevant to your specific interests and given today’s digital organization of research, there is no harm in the existence of lots of (scientifically sound) papers that you find irrelevant. I happen to have 42 International Journal of Communication articles saved in my Zotero library potentially impacting my research compared to 5 from Communication Monographs. But does that mean the International Journal of Communication will just publish anything while Communication Monographs publishes only the highest quality? I don’t think so. There is no way around reading a paper to assess its quality, which can also be subjective, depending on why you are reading the paper (e.g., looking for a clear application of a method vs. a good literature review of a topic etc.).
What certainly remains true is that we should “not use journal-based metrics, such as Journal Impact Factors, as a surrogate measure of the quality of individual research articles, to assess an individual scientist’s contributions, or in hiring, promotion, or funding decisions” (https://sfdora.org/read/). So that tells us what not to use the impact factor and journal-level metrics in general for; but is there any use to them at all? For me, it just helps in structuring the field a bit and can help with journal selection, but if these metrics didn’t exist or weren’t displayed prominently on journal websites, I wouldn’t miss them.
It is also not clear what a “field” is – for instance, I don’t care about the Journal of Advertising and it’s ranked 10th in the “Communication” category. On the other hand, I regularly read papers in Big Data & Society, which is only ranked in “Social Sciences, Interdisciplinary”. On Google Scholar most (clearly social science) communication journals are under “Humanities”. There is a lot of implicit knowledge involved in these things – a publication in a ranked journal can signal that you are part of that community, you are trying to reach people who would also publish there. But an individual paper needs to hold up on its own regardless of where it’s published. Journal metrics are about visibility – although we no longer thumb through the tables of contents of the 20 journals physically available in the library, the journals and their editors still provide orientation and people tend to look to the top-ranked ones.
As in publishing papers in general, there is again a difference between what individuals at different career stages can do and what the ideal system would look like. I think an interesting publishing system for the future would be to preprint everything, somehow organize open peer review, and then have journals and their editorial teams merely compile collections of articles already freely available and basically just paste their seal of approval onto papers for those that need them.