The number of MeSH subject descriptors assigned per PubMed citation by MEDLINE. The figure provides a probability density histogram of the number of MeSH subject descriptors assigned per PubMed citation. The peak (just under a density of 0.1) is at 10 descriptors with a near-normal (skewed slightly towards 0) distribution falling away from that maximum to negligible numbers of descriptors at 1 tag per document and about 26 tags document. The one exception to the smooth distribution is an outlier group at zero descriptors per citation with a density of approximately 0.025.
Good et al. BMC Bioinformatics 2009 10:313 doi:10.1186/1471-2105-10-313