What is the h-index?
The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist’s most cited papers and the number of citations that they have received in other publications. It can also be applied to the productivity and impact of a group of scientists, such as a department or university or country, as well as a scholarly journal.
The index is defined as the maximum value of h such that the given entity (author, journal, department, institution, etc.) has published at least h papers that have each been cited at least h times (https://doi.org/10.1073/pnas.0507655102).
Why use h-index?
Despite being built on the same underlying data as the citations measure, the H indicator returns some different results, these differences are central to the value of h-index. In a large institution producing a lot of research, a research group that is cutting edge can be lost in a citations per paper approach, whereas in h-index analysis, it is the unimportant research that gets overlooked. A small, focused institution is unlikely to compete with a world leading large institution, but can still hold their own. Another approach may have been to replace the citations measure altogether, but the citations measure provides a measure of consistency, rewarding institutions whose performance is solid across the discipline, regardless of whether they have stellar research groups in the mix too. On balance, advisors felt that both indices brought something of value to these observations.
Publication and citation patterns vary dramatically by discipline, which limits their usefulness in overall rankings and h-index is no different. A typical h-index for an academic in Physics will be far higher than that of someone in Sociology, for example. However, when working in a single discipline where differing characteristics by discipline are eliminated, they are more effective and bias is broadly eliminated.
How is it applied?
The h analysis is still based on a dataset which can only be classified by discipline at a journal, rather than article, level. In order to balance for the effects of this and focus on specialists, two h-indices are calculated; one for all the papers that are attributable to the given subject (h0), and one for the papers published in niche journals, that is journals attributable to only one ASJC code within that subject (h1). These are aggregated with double weight given to h1. The results are then scaled and normalized using the same methods applied to the other indicators.