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Elasticsearch: The Definitive Guide [2.x] » Dealing with Human Language » Reducing Words to Their Root Form
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Reducing Words to Their Root Formedit

Most languages of the world are inflected, meaning that words can change their form to express differences in the following:

  • Number: fox, foxes
  • Tense: pay, paid, paying
  • Gender: waiter, waitress
  • Person: hear, hears
  • Case: I, me, my
  • Aspect: ate, eaten
  • Mood: so be it, were it so

While inflection aids expressivity, it interferes with retrievability, as a single root word sense (or meaning) may be represented by many different sequences of letters. English is a weakly inflected language (you could ignore inflections and still get reasonable search results), but some other languages are highly inflected and need extra work in order to achieve high-quality search results.

Stemming attempts to remove the differences between inflected forms of a word, in order to reduce each word to its root form. For instance foxes may be reduced to the root fox, to remove the difference between singular and plural in the same way that we removed the difference between lowercase and uppercase.

The root form of a word may not even be a real word. The words jumping and jumpiness may both be stemmed to jumpi. It doesn’t matter—​as long as the same terms are produced at index time and at search time, search will just work.

If stemming were easy, there would be only one implementation. Unfortunately, stemming is an inexact science that suffers from two issues: understemming and overstemming.

Understemming is the failure to reduce words with the same meaning to the same root. For example, jumped and jumps may be reduced to jump, while jumping may be reduced to jumpi. Understemming reduces retrieval; relevant documents are not returned.

Overstemming is the failure to keep two words with distinct meanings separate. For instance, general and generate may both be stemmed to gener. Overstemming reduces precision: irrelevant documents are returned when they shouldn’t be.

Lemmatization

A lemma is the canonical, or dictionary, form of a set of related words—​the lemma of paying, paid, and pays is pay. Usually the lemma resembles the words it is related to but sometimes it doesn’t — the lemma of is, was, am, and being is be.

Lemmatization, like stemming, tries to group related words, but it goes one step further than stemming in that it tries to group words by their word sense, or meaning. The same word may represent two meanings—for example,wake can mean to wake up or a funeral. While lemmatization would try to distinguish these two word senses, stemming would incorrectly conflate them.

Lemmatization is a much more complicated and expensive process that needs to understand the context in which words appear in order to make decisions about what they mean. In practice, stemming appears to be just as effective as lemmatization, but with a much lower cost.

First we will discuss the two classes of stemmers available in Elasticsearch—Algorithmic Stemmers and Dictionary Stemmers—and then look at how to choose the right stemmer for your needs in Choosing a Stemmer. Finally, we will discuss options for tailoring stemming in Controlling Stemming and Stemming in situ.

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