Today’s article Bornmann L, Daniel HD. What do we know about the h index? J Am Soc Inf Sci Technol. 2007;58(9):1381-1385. Review of research inspired by: Hirsch JE. An index to quantify an individual’s scientific research output. Proc Natl Acad Sci U S A 2005;102(46):16569-16572. Also will allude to: Bornmann L, Daniel HD. The state of h index research. EMBO Rep 2009;10(1):2-6.
What is the h Index? Proposed by Hirsch (2005) Quantifies scientific output of a single researcher as a single number: An h index of 40 means that a scientist has published 40 papers that each had at least 40 citations. “… original and simple new measure incorporating both quantity and visibility of publications”
h index and the Scientific Life Cycle A scientist's h index never decreases It increases as… New high-impact papers are published Sleeping Beauties come to life (van Raan, 2004) A scientist's papers attract citations
Advantages of h Estimates broad impact of cumulative research contributions (Hirsch, 2005) Insensitive to citation extremes Infrequently cited or noncited papers “One-hit wonders” It “favors enduring performers that publish a continuous stream of papers with lasting and above-average impact.”
Article #71 in results list Cited by 72 articles h=71
Article #87 in results list Cited by 87 articles h=87
Cited by 111 articles h=111 Article #111 in results list
Applications of the h Index Micro vs. meso level applications Micro: single researcher Meso (intermediate): group of researchers Scientific facilities Countries (Csajbók et al, 2007) h - b index based on topic or compound search in Web of Science (Banks, 2006) How much work has already been done on topic? What topic is hot or mainstream now?
Convergent Validity of the h Index Hirsch computed h indices for: Nobel Prize physicists in the last 20 years Physicists and astronomers elected to National Academy of Sciences in 2005 Highly cited scientists in biological and biomedical sciences Hirsch’s threshold values Successful scientist: h=20 after 20 years Outstanding scientist: h=40 after 20 years Truly unique individual: h=60 after 20 years, or 90 after 30 years
Disadvantages of WoS-Derived h Publication and citation may actually be lower if multiple authors of same name Or, may be higher (Roediger, 2006) Only journals selected by Thomson Scientific are indexed Unclear citations (e.g., “in press”) Incorrect citations (e.g., wrong first page number) Only journal articles are counted (Also, noted in WoS documentation: subscription depth as factor)
Journal Impact Factors in Tenure and Promotion
Potato, Potahto Simplistic BUTmeasures broad impact of one's work Can’t differentiate between active and inactive scientists BUT never decreases Can’t differentiate between no longer significant works and those now shaping scientific thinking BUTalso can't spotlight works which are "trendy”--this is a bad thing?
Different disciplines, different citation patterns: should be used to compare only scientists of similar professional age working in similar disciplines BUTHirsch himself noted that h in biosciences higher than in physics Combining publication and citation frequencies into one value "posits an equality between two quantities with no evident logical connection" BUT preferable to other single-number criteria (total number of papers; total number of citations; citations per paper)
"Think of two scientists, each with 10 papers with 10 citations, but one with an additional 90 papers with 9 citations each; or suppose one has exactly 10 papers of 10 citations and the other exactly 10 papers of 100 each. Would anyone think them equivalent?” BUT2 individuals with similar hs are comparable in terms of their overall scientific impact, even if their total number of papers or their total number of citations is very different.
Indices that Correct or Complement h One that corrects for self-citations* m quotient: Corrects for bias toward researchers with longer careers/more papers* Ones that look at a definite time period, such as c (for most recent calendar year) hI: Normalizes for different numbers of co-authors in different fields* a and g indexes: More sensitive to highly cited papers—the scientist’s “top performers” * Proposed or developed by Hirsch himself
Conclusion No thorough validation of the h index yet in its various applications Would need to entail… Cross-discipline validation Broad statistical data So, h index and its derivatives and alternatives “should not (yet) be used as a criterion to inform decision making in science…”
What do we know aboutthe h index? Pat Weiss HSLS Journal Club March 4, 2009