Surface plot of covariance between pairs of items in the given test data (DATA-I on the left, and HSQ in the right panel), plotted against item indices. Here only the lower-triangle of the inter-item variance-covariance matrix is plotted, i.e. covariance between the j-th and j′-th item is plotted ∀j^/ ≤ j ; j = 1, 2, … , p; p = 50 for DATA-1 and p = 32 for HSQ. Non-uniformity in the covariance values are displayed in the plots. Outlying inter-item covariance values are parametrised by C, (discussed in Sect. 6.1). For HSQ, C ≈ 20%, while the inter-item covariance sample of DATA-I, causes C to about 8.7%.