There are some methodological issues that are common to all areas of psychology (and most areas of science) that are worth reviewing before plunging into the specific methodological issues of personality research. First is the difference between an observational study and an experimental study. In the former, the variables of interest (e.g. number of temper tantrums and a self-reported measure of irritability) are observed in a group of individuals so that correlations between the variables can be calculated. In the latter, one variable is designated the dependent variable (e.g. number of temper tantrums) and another is designated the independent variable (e.g. number of anger-management therapy sessions). The advantage of an experimental design is that the researcher can be confident that the changes seen in the dependent variable are actually due to changes in that dependent variable.

Most personality research is based on clinical interviews, laboratory experiments, or self-report questionnaires, or some combinations of these. The modern field began with Freud's clinical interviews of psychiatric patients. The advantage of clinical interviews is that they give the researcher a complex, rich view of the person's thoughts and feelings. When combined with behavioral observation, this can give the clinician insights into the personality of the subject that would be unavailable by any other means. The disadvantage is that the data is subject to the interpretive biases of the clinician. A further problem resides in that even with unbiased data it is difficult to reduce the results of the interview into a form suitable for scientific statistical analysis.

Another commonly used method is the laboratory experiment. In laboratory-based research, the variables of interest are precisely determined and the conditions under which experimental participants can behave are tightly controlled. As a consequence, the results of laboratory research are much easier to analyze and interpret than the results of interview-based research and are less subject to the influence of the experimenter's bias. An example of a laboratory-based study of personality is one in which subjects are shown an upsetting film and their physiological, subjective, and behavioral responses are measured. With some caveats, individual differences in response can then be attributed to differences in personality among the subjects. /PARAGAPH The last major technique in personality psychology is the self-report questionnaire. Subjects are given a series of questions about their typical behaviors, thoughts, and feelings. Answers to the questions are then scored to determine personality variables. The variables of interest can range from the very specific (e.g. current level of disgust) to the very broad (e.g. extroversion). Self-report questionnaires are used in almost every area of personality psychology, but they play a central role in trait theory. Trait theory is based on factor analysis of a large number of questionnaires about personality given to large numbers of subjects. The result of such factor analysis is a small set of composite variables that are supposed to reflect basic traits. Each individual's scores on these traits can then be correlated with various behaviors, thoughts, and feelings that those individuals may exhibit.