People differ from one another and individuals change over time. Some people are more intelligent than other people. Some people are less likely to become criminals that other people. Some people like to drink beer, others don't. People have differences of opinion. They wear different kinds of cloths, and so on. Further, how a person behaves can change over time. For example, a person's opinion about something can change. People tend to wear different types of clothes at different points in time. These "individual differences" are the stuff of psychological inquiry.

When psychologists do research they are simply trying to account for these individual differences. Why do some people become depressed while others seem to have a sunny outlook on life? Why do some students do well in school while others struggle? Why do some people vote and others don't. The list of different kinds of behaviors that interest us is endless. The immediate purpose of any psychological research project is to show how individual differences are related to other factors (events, objects, states of mind, previous history, social situations, genes, etc.). For example, a psychologist might what to know if academic performance is related to study time. Technically, this researcher is asking if variability associated with academic performance (e.g. grades) can be related in some systematic way to differences in study time.

The first thing that a psychologist has to do in order to study variability in behavior is to measure the behavior to be studied (usually referred to as the dependent variable). The psychologist also has to measure those things that the psychologist thinks are related to the behavior of interest (I will refer to these factors as predictor variables). Once these variables have been measured, the psychologist has to come up with some way to show that changes in the targeted behavior are related to changes in the predictor variable. Here is where psychologists employ various research methods and statistical procedures. These methods and procedures are simply tools that psychologists use to link differences in behaviors to differences in predictor variables.

The important thing to remember is that psychologists study individual differences. If people did not differ from one another, psychologists would be out of business! Variability in behavior is the currency of our trade.