We've been examining whether or not to restore stopwords to the SearchWorks index. Stopwords are words ignored by a search engine when matching queries to results. Any list of terms can be a stopword list; most often the stopwords comprise the most commonly occurring words in a language, occasionally limited to certain functions (articles, prepositions vs. verbs, nouns).
The original usage of stopwords in search engines was to improve index performance (query matching time and disk usage) without degrading result relevancy (and possibly improving it!). It is common practice for search engines to employ stopwords; in fact Solr (http://lucene.apache.org/solr), the search engine behind SearchWorks, has English stopwords turned on as the default setting.
In our implementation of SearchWorks, there was no compelling reason to change most of the default Solr settings; thus, since SearchWorks's inception we have been using the following stopword list: a, an, and, are, as, at, be, but, by, for, if, in, into, is, it, no, not, of, on, or, s, such, t, that, the, their, then, there, these, they, this, to, was, will, with.
What follows is an analysis of how stopwords are currently affecting SearchWorks, and what might happen if we restore stopwords to SearchWorks, making every word signficant for every search.