MeSH: Hierarchical List

This post examines the Hierarchical List of the Medical Subject Headings (MeSH).

MeSH descriptors are organized in 15 categories. For example, category A for anatomic terms, category B for organisms, C for diseases, D for drugs and chemicals, etc. Each category is further divided into subcategories and within each subcategory, descriptors are displayed hierarchically from the broadest to the most specific.

Because of the branching structure of the hierarchies, these lists are sometimes referred to as "trees". Each descriptor appears in at least one place in the hierarchy but may also appear in other categories. A good example is Breast Neoplasms. This descriptor appears in the Diseases category 'Neoplasms' in the subcategory 'Neoplasms by Site'. It also appears in the Diseases category 'Endocrine Diseases' in the subcategory 'Breast Diseases'. Here is what it looks like:



Why is this important?

The subject indexers are instructed to find and use the most specific MeSH descriptor that is available to represent each indexable concept. For example, articles concerning Streptococcus pneumoniae will be found under the descriptor Streptococcus Pneumoniae rather than the broader term Streptococcus, while an article referring to a new streptococcal bacterium which is not yet in the vocabulary will be listed directly under Streptococcus.

It makes sense then that the searcher would consult the trees as well. Here's where you find out how your topic fits in the hierarchical structure and whether you need to select a broader or more specific descriptor to tease out the most relevant articles.