linear regression and correlation coefficient for two quantitative variables (Sect. 3.2);
tabular regression and correlation ratio for the mixed scale case (Sect. 3.8.3); and
contingency table, Quetelet index, statistical independence, and Pearson’s chi-squared for two nominal variables; the latter is treated as a summary correlation measure, in contrast to the conventional view of it as just a criterion of statistical independence (Sect. 3.6.1); moreover, a few less known least-squares based concepts are outlined, including canonical correlation and correspondence analysis.