The H-index, proposed by physicist J.E. Hirsch (hence the H) in 2005, is a way to measure the individual academic output of a researcher.
"A scientist has index h if h of his or her Np papers have at least h citations each and the other (Np-h) papers have ≤h citations each" (Hirsch, 2005).
To determine the H-index of a researcher you need the list of the publications of that researcher and the number of citations each publication received. Then sort the publication list by the number of citations: the publication with the highest number of citations is number 1. The H-index is the number where the number of the publication in the list and the number of citations received are the same.
Example of an H-graph in Scopus (click on the picture to enlarge it)
The H-index has become a popular performance indicator, probably because the calculation of the h-index is easy to understand, and because it takes into account both productivity (the number of publications) and academic impact (the number of citations received). The H-index of an author is also easy to find: in Scopus, Web of Science and Google Scholar within a few mouseclicks (but be aware, that's the 'quick and dirty approach').
However, the H-index has various weak points. An easy example: the H-index doesn’t take into account the age of the researcher, which makes it unfair to use the H-index to compare two researchers in different stages of their research career. A professor at the end of his career has published more than a PhD-student and has had more time to get cited. There are also examples of researchers changing disciplines, taking their high H-index with them to a discipline with in general lower H-indexes. Because of differences in publication cultures between disciplines, you can’t compare the H-index of researchers from different disciplines.
Examples of researchers changing disciplines
Hirsch, J. E. (2005). An index to quantify an individual's scientific research output. Proceedings of the National Academy of Sciences of the United States of America, 102(46), 16569-16572. https://doi.org/10.1073/pnas.0507655102
To create the list of publications and the number of citations received you can use different sources. The most used sources are Scopus, Web of Science and Google Scholar. Which source you use, determines the H-index you find for a particular researcher, because the ‘publication universes’ of the sources are different. For example Google Scholar also indexes university repositories, so citations in student’s theses are counted as well.
The example below shows the general picture: the H-index based on publications in Web of Science is lower than the H-index based on publications in Scopus; the H-index in Google Scholar is by far the highest.
An example: Arnold Bakker, Professor of Work and Organizational Psychology of the Erasmus School of Social and Behavioural Sciences (ESSB).
Our advice: When you are looking for your own H-index or the H-index of another researcher, don’t rely on easily-available numbers. The basis of the H-index is the publications list: you have to check whether the publications in the list you use are indeed of the researcher and whether publications might be missing. Always give information about the source used and use, when possible, multiple sources.