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optimal matching : ウィキペディア英語版
optimal matching
Optimal matching is a sequence analysis method used in social science, to assess the dissimilarity of ordered arrays of tokens that usually represent a time-ordered sequence of socio-economic states two individuals have experienced. Once such distances have been calculated for a set of observations (e.g. individuals in a cohort) classical tools (such as cluster analysis) can be used. The method was tailored to social sciences〔A. Abbott and A. Tsay, (2000) ''(Sequence Analysis and Optimal Matching Methods in Sociology: Review and Prospect )'' Sociological Methods & Research], Vol. 29, 3-33. 〕 from a technique originally introduced to study molecular biology (protein or genetic) sequences (see sequence alignment). Optimal matching uses the Needleman-Wunsch algorithm.
== Algorithm ==
Let S = (s_1, s_2, s_3, \ldots s_T) be a sequence of states s_i belonging to a finite set of possible states. Let us denote the sequence space, i.e. the set of all possible sequences of states.
Optimal matching algorithms work by defining simple operator algebras that manipulate sequences, i.e. a set of operators a_i: \rightarrow . In the most simple approach, a set composed of only three basic operations to transform sequences is used:
* one state s is inserted in the sequence a^_ (s_1, s_2, s_3, \ldots s_T) = (s_1, s_2, s_3, \ldots, s', \ldots s_T)
* one state is deleted from the sequence a^_ (s_1, s_2, s_3, \ldots s_T) = (s_1, s_3, \ldots s_T) and
* a state s_1 is replaced (substituted) by state s'_1, a^_ (s_1, s_2, s_3, \ldots s_T) = (s'_1, s_2, s_3, \ldots s_T).
Imagine now that a ''cost'' c(a_i) \in ^+_0 is associated
to each operator. Given two sequences S_1 and S_2,
the idea is to measure the ''cost'' of obtaining S_2 from S_1
using operators from the algebra. Let A= be a sequence of operators such that the application of all the operators of this sequence A to the first sequence S_1 gives the second sequence S_2:
S_2 = a_1 \circ a_2 \circ \ldots \circ a_ (S_1) where a_1 \circ a_2 denotes the compound operator.
To this set we associate the cost c(A) = \sum_^n c(a_i), that
represents the total cost of the transformation. One should consider at this point that there might exist different such sequences A that transform S_1 into S_2; a reasonable choice is to select the cheapest of such sequences. We thus
call distance
d(S_1,S_2)= \min_A \left \
that is, the cost of the least expensive set of transformations that turn S_1 into S_2. Notice that d(S_1,S_2) is by definition nonnegative since it is the sum of positive costs, and trivially d(S_1,S_2)=0 if and only if S_1=S_2, that is there is no cost. The distance function is symmetric if insertion and deletion costs are equal c(a^) = c(a^); the term ''indel'' cost usually refers to the common cost of insertion and deletion.
Considering a set composed of only the three basic operations described above, this proximity measure satisfies the triangular inequality. Transitivity however, depends on the definition of the set of elementary operations.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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