CLN - Ask The Expert

Scientific Impact and the H-Index

Ask the Expert: September 2019

William E. Schreiber, MD

What is the h-index?

The h-index is a means of measuring scientists’ impact on their field, determined from two quantities: the number of publications by a scientist and the number of times those publications have been cited. Think of it as the intersection of productivity (papers published) and recognition (citations). The higher the h-index, the greater a scientist’s academic footprint.

In what ways does the h-index matter?

Jorge Hirsch, a physicist, created this index in 2005. Since then, scientists in other fields have begun using it too, as of course many scientists and other academics compete for promotions, research grants, and professional awards. The committees and review panels that hand out these distinctions expect to see an applicant’s record of accomplishment, and much of that evidence is in peer-reviewed journals and other publications.

Assigning importance to individual publications is a subjective, time-consuming process. In contrast, the h-index is determined directly from data available through the internet. No subjective judgment is required, and it is easy to compare an h-index number among individuals and against established benchmarks.

How would a researcher calculate this number?

Identify all your publications, then look up the number of citations for each paper on Google Scholar, Web of Science, or Scopus (the latter two databases require a subscription). Order the publications from highest to lowest number of citations. Then, go down the list until the number of citations for a paper is less than the number of papers you have counted. The h-index is equal to the number of papers (h) that have been cited at least that many times each.

What is a good h-index?

That is a matter of opinion. A colleague and I surveyed typical values for academic physicians in 14 medical specialties (Am J Clin Pathol 2019;151:286-91). We found that, on average, assistant professors have an h-index of 2-5, associate professors 6-10, and full professors 12-24. These are mean or median values only—the distribution of values at each rank is very wide. If you hope to win a Nobel Prize, your h-index should be at least 35 and preferably closer to 70.

Is it really that simple to characterize a scientist’s achievements?

Although simple to calculate, the h-index does not capture the full story of a scientist’s contributions. For example, single author papers count the same as multi-author papers. And there is no extra credit for being the first or last author, which usually indicates a greater role in the project.

In addition, self-citation can inflate one’s score. And the h-index never decreases, so it is not a good indicator of recent activity. Most reviewers consider the h-index as one piece of a larger picture, rather than a definitive ranking tool.

The h-index doesn’t involve lab medicine. Why should I care about it?

If you belong to a university department, you are likely to hear about quantitative measures of academic performance. By understanding the h-index, you are in a better position to evaluate its pros and cons as a yardstick for scientific achievement. This will be helpful when evaluating the work of professional colleagues or when arguing your own case for promotion.

William E. Schreiber, MD, is a professor of pathology and laboratory medicine at The University of British Columbia and clinical director of chemistry at LifeLabs in Burnaby, British Columbia. E-mail: [email protected]