  Background: Citation analysis has become an important tool for research
performance assessment in the medical sciences. However, different areas of
medical research may have considerably different citation practices, even
within the same medical field. Because of this, it is unclear to what extent
citation-based bibliometric indicators allow for valid comparisons between
research units active in different areas of medical research.
  Methodology: A visualization methodology is introduced that reveals
differences in citation practices between medical research areas. The
methodology extracts terms from the titles and abstracts of a large collection
of publications and uses these terms to visualize the structure of a medical
field and to indicate how research areas within this field differ from each
other in their average citation impact.
  Results: Visualizations are provided for 32 medical fields, defined based on
journal subject categories in the Web of Science database. The analysis focuses
on three fields. In each of these fields, there turn out to be large
differences in citation practices between research areas. Low-impact research
areas tend to focus on clinical intervention research, while high-impact
research areas are often more oriented on basic and diagnostic research.
  Conclusions: Popular bibliometric indicators, such as the h-index and the
impact factor, do not correct for differences in citation practices between
medical fields. These indicators therefore cannot be used to make accurate
between-field comparisons. More sophisticated bibliometric indicators do
correct for field differences but still fail to take into account within-field
heterogeneity in citation practices. As a consequence, the citation impact of
clinical intervention research may be substantially underestimated in
comparison with basic and diagnostic research.
