As a rule, a large random sample is more likely to be representative than a small one. Some surveys are based on unobtrusive measures, in which the subjects never know that they have been included in a research project. The lost-letter technique is a modified unobtrusive measure because the investigator presents potential subjects with a specific stimulus. Most unobtrusive measures provide little knowledge of the sample of subjects. The case study describes one person, a group of people, or an event in detail.
Extensive information is gained through a series of interviews.
Psychological tests also are useful for gathering information. The case history includes social, educational, and medical information, as well as the psychological background, obtained from all available sources. Case histories, interviews, and psychological tests may involve idiographic analysis, concerned with discovering the underlying principles in a given case. They also may involve nomothetic research, which has the purpose of discovering general laws of behavior. In the classical experiment the investigator manipulates an independent variable, usually a stimulus, and notes its influence on the dependent variable, usually a response.