Table 1.

Summary of key features of database hierarchies

Medical Subject Headings	Emtree
• Consists of sets of terms naming descriptors in an 11-level hierarchical structure that permits searching at various levels of specificity	• 15 Major branches or facets • The largest branch covers physical diseases, disorders and abnormalities
• Provides consistent way to retrieve information that may use different terminology for the same concepts
• Descriptors arranged in both alphabetical and hierarchical structure. At the most general level of the hierarchical structure there are very broad headings. More specific headings are found at more narrow levels	• Includes all MeSH terms, but also has its own unique features, including the use of more natural language to describe terms and the use of many synonyms and spelling variants