Psychologists have many ways of studying behavior. The many branches of psychology, and the many approaches within each branch, have given birth to multiple approaches. Three ways of studying human behavior that an everyday person may encounter are longitudinal, cross-sectional, and case-control studies.
In a longitudinal study, a psychologist recruits a group of people, often but not always children, and studies them through time. For example, he or she might compare a group of children who score high on a particular IQ test with a group with mid-range scores.
By mailing or e-mailing subjects a questionnaire each year, or through regular interviews, the psychologist tries to examine the life experiences of each group.
He or she might ask each year if they are married, how many children they have, and what their income is. A questionnaire might ask about attendance at religious services or about physical health problems. The psychologist is looking for common factors, and the ways that they may change through time.
A problem with studies of this kind is that people drop out. Psychologists suspect that the people who drop out may have characteristics in common, which would lead to a study’s failure to examine a truly representative sample.
A longitudinal study is an observational study. The psychologist is not affecting the group under study, except by observing them.
A cross-sectional study is also an observational study. Unlike a longitudinal study, it examines a group of people at one point in time. In this kind of research, a psychologist is trying to gather information about present conditions, rather than about the effects of a certain characteristic or circumstance through time.
A psychologist might look for an immediate relationship, a correlation, for example between weeks unemployed and hours spent exercising. Often, as with longitudinal studies, researchers must rely on what their subjects tell them.
Another possible source of inaccuracy is the source of the researcher’s information. Some material for cross-sectional studies was originally collected by other researchers. For example, a psychologist might use U.S. Census data, with names and all identifying characteristics removed. Thus, a researcher sometimes cannot control the selection of subjects, or the manner in which the information was gathered.
In a case-control study, a group of people with a particular characteristic is matched with another group of people who are similar in many ways, but generally lack the characteristic being studied.
The most famous case-control study is probably the one that compared people with lung cancer to similar people who did not have it. The major difference turned out to be that those with lung cancer tended to be cigarette smokers. This is no longer news, but was actually quite controversial at the time. In fact, the observation that smokers are more likely to have lung cancer does not prove that smoking caused it. It’s a pretty good clue for a smoker, though.
The point is that case-control studies can show that a behavior and a characteristic are linked, but cannot show that one caused the other. Psychologists and other scientists express this truth by saying, “correlation is not causation.”
Psychologists research human behavior with many kinds of studies. They must always let the subjects they gather know that they are under study, and warn them of possible bad effects as a result of being studied.
Psychologists are also required to keep any personal information about a subject strictly private. Any information a study gathers is only to be used to further an investigation, and to increase humanity’s store of knowledge.
Handbook of Psychology, Research Methods in Psychology, by John Schinka and others
Fundamental Statistics in Psychology and Education by J. P.Guildford