Factor analysis (FA) has somewhat similar aims to principal component analysis (PCA) in that it is a variable-directed technique which is appropriate when the variables arise 'on an equal footing'. The basic ideas of FA were suggested around the turn of the century by Francis Galton and Charles Spearman among others, and originated mainly from the efforts of psychologists to gain a better understanding of 'intelligence'. Intelligence tests customarily contain a large variety of questions which depend to a greater or lesser extent on verbal ability, mathematical ability, memory, and so on. Most applications of FA have been in psychology and the social sciences. In the early days of FA, a variety of iterative methods were used to estimate the factor loadings. These involved subjective judgement, such as guessing the communalities, with the result that different researchers could analyse the same data and find entirely different factors. One popular method was the principal-factor method.