Factor analysis
Factor analysis was built upon the earlier development of the correlation coefficient, which provides a numerical estimate of the degree to which variables are "co-related". For example, if "frequency of exercise" and "blood pressure level" were measured on a large group of people, then intercorrelating these two variables would provide a quantitative estimate of the degree to which "exercise" and "blood pressure" are directly related to each other. Factor analysis performs complex calculations on the correlation coefficients among the variables within a particular domain (such as cognitive ability or personality trait constructs) to determine the basic, unitary factors underlying the particular domain.