“My research: > 185 papers, h-index 40.” A random quote from a curriculum vitae in the World Wide Web. Sometimes, researchers love their Hirsch index, better known as the h-index. But what does the measure actually mean? Is it a reliable indicator of scientific impact?
Our colleagues Ludo Waltman and Nees Jan van Eck have studied the mathematical and statistical properties of the h-index. Their conclusion: the h-index can produce inconsistent results. For this reason, it is actually not the reliable measure of scientific impact that most users think it is. As a leading scientometric institute, we have therefore published the advice to all universities, funders, and academies of science to abandon the use of the h-index as a measure of the overall scientific impact of researchers or research groups. There are better alternatives. The paper by Waltman and Van Eck is now available as a preprint and will soon be published by the Journal of the American Society for Information Science and Technology JASIST.
The h-index is a measure of a combination of productivity and citation impact. It is calculated by ordering the number of publications by a particular researcher on the basis of the total number of citations they have received. For example, someone who has an h-index of 40 has published at least 40 articles that have each been cited at least 40 times. Moreover, the remaining articles have not been cited more than 40 times each. The higher the h-index the better.
The h-index was proposed by physicist Jorge Hirsch in 2005. It was an immediate hit. Nowadays, there are about 40 variants of the h-index. About one quarter of all articles published in the main scientometric journals have cited Hirsch’ article in which he describes the h-index. Even more important has been the response by scientific researchers using the h-index. The h-index has many fans, especially in the fields that exchange many citations, such as the biomedical sciences. The h-index is almost irrresistable because it seems to enable a simple comparison of the scientific impact of different researchers. Many institutions have been seduced by the siren call of the h-index. For example, the Royal Netherlands Academy of Arts and Sciences (KNAW) in the Netherlands inquires about the value of the h-index in its recent forms for new members. Individual researchers can look up their h-index based on Google Scholar documents via Harzing’s website publish or perish. Both economists and computer scientists have produced a ranking of their field based on the h-index.
Our colleagues Waltman and Van Eck have now shown that the h-index has some fatal shortcomings. For example, if two researchers with a different h-index co-author a paper together, it may lead to a reversal of their position in an h-index based ranking. The same may happen when we compare research groups. Suppose we have two groups and each member of group A has a higher h-index than a paired researcher in group B. We would now expect that the h-index of group A as group is also higher than that of group B. Well, that does not have to be the case. Please note that we are now speaking of a calculation of the h-index based on a complete and reliable record of documents and citations. The problematic nature of the data if one uses Google Scholar as data source is a different matter. So, even when we have complete and accurate data, the h-index may produce inconsistent results. Surely, this is not what one wants using the index for evaluation purposes!
At CWTS, we have therefore drawn the conclusion that the h-index should not be used as measure of scientific impact in the context of research evaluation.