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

5.4. Correspondence Analysis 105Figure 5.6. Correspondence analysis plot for summer species at Irish wetlandsites. The symbol × denotes site; ◦ denotes species.5.4.1 ExampleFigure 5.6 gives a plot obtained by correspondence analysis for a data setthat recorded the presence or otherwise of 52 bird species at a number ofwetland sites in Ireland. The data displayed in Figure 5.6 refer to summersightings and are part of a much larger data set. (The larger set waskindly supplied by Dr. R.J. O’Connor of the British Trust for Ornithology,and which was analysed in various ways in two unpublished studentprojects/dissertations (Worton, 1984; Denham, 1985) at the University ofKent.) To avoid congestion on Figure 5.6, only a few of the points correspondingto sites and species have been labelled; these points will nowbe discussed. Although correspondence analysis treats the data differentlyfrom a biplot, it is still true that sites (or species) which are close to eachother on the correspondence analysis plot are likely to be similar with respectto their values for the original data. Furthermore, as in a biplot, wecan interpret the joint positions of sites and species.On Figure 5.6 we first note that those sites which are close to each otheron the figure also tend to be close geographically. For example, the groupof sites at the top right of the plot {50, 53, 103, 155, 156, 235} are all inland