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Frequently used words with limited semantic specificity can be excluded from search indexes to improve the speed and specificity of searches. Examples of English words typically included in "stop word" lists include: a, an, and, as, at, be, by … of … the, etc.

General purpose search software often includes a default list of "stop words" but these may not be entirely appropriate for SNOMED CT searches. For example, the MySQL default "Full-Text Stopwords" includes "no", "non", "not", "without" and many other words which may be highly significant when searching for clinical terms.

The SNOMED CT International Release includes a suggested "stop list" (referred to as the ExcludedWords Table). This is only available in English and is the list of exclusions used when generating keyword lists distributed with the same release.

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Figure 14Constraining searches (for words in any order) by excluding stop words