International
Tables for Crystallography Volume H Powder diffraction Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk © International Union of Crystallography 2018 
International Tables for Crystallography (2018). Vol. H, ch. 3.8, p. 327
Figure 3.8.2^{a} 

Figure 3.8.2
Four different methods of estimating the number of clusters present in a set of 23 powder patterns for the drug doxazosin. A total of five polymorphs are present, as well as two mixtures of these polymorphs. (a) A scree plot from the eigenvalue analysis of the correlation matrix; (b) the use of the C test (the coefficients have been multiplied by 100.0), which gives an estimate of five clusters using its local maximum. The γ test estimates that there are seven clusters and the CH test has a local maximum at seven clusters. Numerical details are given in Table 3.8.2. 