Jolliffe I. Principal Component Analysis (2ed., Springer, 2002)(518s)

9.2. Cluster Analysis 215Figure 9.3. Aphids: plot with respect to the first two PCs showing four groupscorresponding to species.to verify that a given dissection ‘looks’ reasonable, rather than to attemptto identify clusters. An early example of this type of use was given by Moserand Scott (1961), in their Figure 9.2. The PCA in their study, which hasalready been mentioned in Section 4.2, was a stepping stone on the wayto a cluster analysis of 157 British towns based on 57 variables. The PCswere used both in the construction of a distance measure, and as a meansof displaying the clusters in two dimensions.Principal components are used in cluster analysis in a similar mannerin other examples discussed in Section 4.2, details of which can be foundin Jolliffe et al. (1980, 1982a, 1986), Imber (1977) and Webber and Craig(1978). Each of these studies is concerned with demographic data, as is theexample described next in detail.Demographic Characteristics of English CountiesIn an unpublished undergraduate dissertation, Stone (1984) considereda cluster analysis of 46 English counties. For each county there were 12