Factor analysis is an important statistical method for assessing the construct validity of the underlying dimensions of clinical phenomena. It is especially useful in psychiatric and social research where the observed phenomenon may include multiple variables that may have varying correlations with each other. It is also a key technique in the psychometric validation of new questionnaires. As with any statistical technique the results from factor analysis are only valid if the methods used to collect the data are based on sound epidemiological methods. The reader is encouraged to scrutinise methods of data collection as well as the interpretation of study findings.