계층군집화

• 참조블로그: http://m.blog.daum.net/etineye/20
• Ward linkage: 두 개의 cluster를 묶을 때, SSE가 최소화 되는 방향으로 선택.

hclust {stats}

• http://stat.ethz.ch/R-manual/R-devel/library/stats/html/hclust.html
the agglomeration method to be used. This should be (an unambiguous abbreviation of) one of "ward.D", "ward.D2", "single", "complete", "average" (= UPGMA), "mcquitty" (= WPGMA), "median" (= WPGMC) or "centroid" (= UPGMC).
• Ward clustering: Two different algorithms are found in the literature for Ward clustering. The one used by option "ward.D" (equivalent to the only Ward option "ward" in R versions <= 3.0.3) does not implement Ward's (1963) clustering criterion, whereas option "ward.D2" implements that criterion (Murtagh and Legendre 2014). With the latter, the dissimilarities are squared before cluster updating. Note that agnes(*, method="ward") corresponds to hclust(*, "ward.D2").