The h-index is a commonly used measure that aims to encapsulate a scientist's productivity and impact into a single number. The metric is named for physicist J.E. Hirsch, who came up with the index in 2005 as a useful way to "characterize the scientific output of a researcher". The index takes into account the distribution of citations that a researcher's publications have received. As Wikipedia puts it: "a scholar with an index of h has published h papers each of which has been cited in other papers at least h times". This can be visualized if we plot based on decreasing number of citations of a scientist's papers:
Although the h-index has its merits as a quick and easy metric, there are a number of things the h-index does NOT tell you:
1. The stage of your career.
Are you a newly minted assistant professor with a few highly-cited papers? Or have you been at this for years, with more moderate citation levels? As shown below, two researchers could have the same h-index, but drastically different citation records. This illustrates the fact that h-index is limited by the total number of publications. Although Researcher A's work has been highly cited, his/her h-index is limited to 4.
2. Whether you were first author or fourteenth on that groundbreaking paper.
The h-index only cares whether your name is on the paper or not. This could skew things in favor of big-name researchers who are put on many publications as collaborators.
3. Whether citations are due to positive or negative references.
In other words, maybe a paper is getting cited a lot because it is controversial or has since been disproven.
4. The impact factors of the journals you have published in.
The h-index doesn't take impact factor into account - rather, it is only concerned with the number of citations to your papers. This is a good thing, as we've previously discussed why impact factor is an inadequate measure. In addition, since impact factor is calculated as an average of all of a journals' articles, it can't tell us much about the impact of a single paper.
5. How active you are in the scientific community.
What "scientific outputs", other than publications in scientific journals, are you contributing? Do you communicate science to the general public? Are you involved in scientific outreach? Are you a good mentor who is training the next generation of great scientists? Do you contribute to the discussion and dissemination of research? How about your participation in peer review??
Beyond the h-index
While Hirsch may have designed the h-index with the noble intentions of measuring both quantity and quality of scientific output, it's clear that there are a number of key flaws. However, that's not to say that the h-index can't be useful as a rough measure - so long as whoever is evaluating you (be it your tenure committee, grant reviewers, etc.) digs deeper and looks at the underlying data.
Want to be seen as more than just your h-index? Start getting credit for all your peer review contributions today:
Alicia is a neuroscience graduate student in California. She's into health, tech, and science communication, and is super excited to write about peer review and academic publishing for the Publons blog. You can find her on Twitter @AliciaShiu.