Morphology refers to the derivation of a word’s root by analyzing suffixes and prefixes. This process is different from stemming, because it utilizes knowledge of the language and roots to drive a more accurate analysis.
ATG Search uses a practical definition of “root” as the base form of a word that is effective for retrieval. Since this is not necessarily the etymological root word, ATG Search calls this an index term to avoid confusion. ATG Search is selective, and therefore not uniform in the amount of morphology it does, if it results in improved search performance. For example, in general, ATG Search derives er forms down to their verb roots, such as attackerattack+er, but some er forms are better left as index terms, such as user and computer.
There are two levels of morphological analysis:
Inflectional morphology involves the simple endings associated with plural nouns (such as book +s), verb tense (sell +ing), and adjectival or adverbial forms (such as happy +er). This level of analysis can be closely approximated by stemming.
Derivational morphology involves more complex affixes that transform a word into another word. For example, some suffixes turn verbs into nouns (acquire +ionacquisition) and some suffixes turn nouns and adjectives to verbs (simple +fysimplify, summary +izesummarize). Prefixes typically add meaning to words (un+ happyunhappy, re+ playreplay).
A word can include both inflectional and multiple derivational endings.
ATG Search performs morphology on both the query and the documents. Doing morphology on both index and query terms greatly increases the matches between a query term and indexed documents, increasing retrieval results and improving the ranking of results. Alternatively, you could index by literal form and use morphology to expand the query terms into all possible forms, similar to the process described in the stemming section.
The default behavior of ATG Search is to reduce query terms down to their index terms and use those for retrieval. The Literal Constraint section of the User-Entered Query Operators chapter describes how to find only the literal form of a query term. ATG Search records morphology information so that terms can be retrieved by their literal form.
Note: Since ATG Search allows custom terminology, administrators can alter what ATG Search considers an index term, such as adding seller so that it doesn’t get analyzed to sell. See the Custom Terminology section of this chapter.
