lemma与stem的区别
Difference between stem and lemma
先从wikipedia上看看什么是stem，什么是lemma？
Lemma(morphology)：In morphology and lexicography, a lemma (plural lemmas or lemmata) is the canonical form, dictionary form, or citation form of a set of words(headword). In English, for example, run, runs, ran and running are forms of the same lexeme, with run as the lemma. Lexeme, in this context, refers to the set of all the forms that have the same meaning, and lemma refers to the particular form that is chosen by convention to represent the lexeme. In lexicography, this unit is usually also the citation form or headword by which it is indexed. Lemmas have special significance in highly inflected languages such as Turkish and Czech. The process of determining the lemma for a given word is called lemmatisation.
word stem：In linguistics, a stem is a part of a word. The term is used with slightly different meanings. In one usage, a stem is a form to which affixes can be attached. Thus, in this usage, the English word friendships contains the stem friend, to which the derivational suffix -ship is attached to form a new stem friendship, to which the inflectional suffix -s is attached. In a variant of this usage, the root of the word (in the example, friend) is not counted as a stem.In a slightly different usage, which is adopted in the remainder of this article, a word has a single stem, namely the part of the word that is common to all its inflected variants.Thus, in this usage, all derivational affixes are part of the stem. For example, the stem offriendships is friendship, to which the inflectional suffix -s is attached.
Difference between stem and lemma：
Stem is the part of the word that never changes even when morphologically inflected, whilst a lemma is the base form of the word. For example, from "produced", the lemma is "produce", but the stem is "produc-." This is because there are words such as production.In linguistic analysis, the stem is defined more generally as the analyzed base form from which all inflected forms can be formed. When phonology is taken into account, the definition of the unchangeable part of the word is not useful, as can be seen in the phonological forms of the words in the preceding example: "produced" /prəˈdjuːst/ vs. "production" /prəˈdʌkʃən/.Some lexemes have several stems but one lemma. For instance "to go" (the lemma) has the stems "go" and "went". (The past tense is based on a different verb, "to wend". The "-t" suffix may be considered as equivalent to "-ed".)
从上面我们可以看出，lemma一般是指词型的还原，一般就是一个结果，而stem是词干，根据不同的定义略微不同，下面我们看下使用程序分析的结果，其中lemma使用stanford的NLP工具，stem使用NLTK包中的stem（snow，porter，lancaster三个算法）
原句：This work shows that single and double Ala substitutions of His18 and Phe21 in IL-8 reduced up to 77-fold the binding affinity to IL-8 receptor subtypes A (CXCR1) and B (CXCR2) and to the Duffy antigen.
lemma：this work show that single and double alum substitution of his18 and phe21 in il-8 reduce up to 77-fold the binding affinity to il-8 receptor subtype a -lrb- cxcr1 -rrb- and b -lrb- cxcr2 -rrb- and to the duffy antigen .
snowstem：this work show that singl and doubl ala substitut of his18 and phe21 in il-8 reduc up to 77-fold the bind affin to il-8 receptor subtyp a ( cxcr1 ) and b ( cxcr2 ) and to the duffi antigen .
porterstem：Thi work show that singl and doubl Ala substitut of His18 and Phe21 in IL-8 reduc up to 77-fold the bind affin to IL-8 receptor subtyp A ( CXCR1 ) and B ( CXCR2 ) and to the Duffi antigen .
lancasterstem：this work show that singl and doubl ala substitut of his18 and phe21 in il-8 reduc up to 77-fold the bind affin to il-8 receptor subtyp a ( cxcr1 ) and b ( cxcr2 ) and to the duffi antigen .