There are a number of methods of collecting data and making observations. Case studies attempt to reconstruct a person's life in depth to understand how patterns of behavior have developed.
Psychologists often employ standardized personality, aptitude and intelligence tests.
The controlled experiment allows a psychologist to test a hypothesis about the relationship of the two variables to see if there is a cause-effect relationship. For instance a survey in Cosmopolitan Magazine on premarital sex activity will probably not represent the general U.S. population but a subgroup of relatively affluent, young, white women. By comparing results of the experimental group to those of the control group the researcher can assess whether significant change occurred by the treatment. The double-blind procedure controls for this subject bias. Most scores cluster around the center with lesser number of scores being higher or lower.