Keyword systems work best when the you specify words that are highly selective--Words that occur infrequently in the whole collection of documents you are looking for, but occur frequently in the documents you would be interested in.
Words of low selectivity are often called stop words. Examples include "is," "to," "and," "the," or other frequently used words.
Even fairly specific terms can be stop words. For example, in the documents of a steel company,"steel" would be a stop word because it shows up frequently.
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