The h-index simultaneously measures productivity and academic influence by providing a single value as a ratio of the number of outputs and number of citations e.g. if an author has 10 publications with 10 or more citations, they will have a h-index of 10.
The h-index must be used with caution:
- It only becomes meaningful once a researcher has a reasonable number of publications and citations.
- It can never be higher than the number of outputs. If the author has one publication and the publication has 100 citations, the author's h-index will still be 1.
- It can never be higher than the number of citations received by the most cited paper, regardless of the amount of output. If an author has 20 publications and each publication only has a single citation, the h-index will still be 1.
- It increases with time (or stays the same). It cannot go down.
Variations of the h-index, intended to address limitations of the h-index, are the g-index and the m-index:
- The g-index gives extra weighting to the most cited publications. The g-index is always the same or higher than the h-index.
- The m-index displays the h-index per year since first publication. The m-index is useful for comparing researchers within a field with very different career lengths.
The h-index is usually calculated for individual researchers but can also be calculated for other entities such as a research group or a journal.
Citation behaviour varies between disciplines. As a non-field normalised metric, the h-index should only be used to benchmark entities in similar disciplines.
The h-index will vary for the same research entity in different database sources. Comparisons should therefore only be made from a single source. See appropriate use of publication and citation metrics.
Recommended sources for calculating the h-index, the g-index and the m-index are listed below.