The text describes the basic methods. You should also be able to describe limitations of each method. Given an example of a correlational study about variables that you are familiar with you should be able to determine whether there is a positive correlation, negative correlation, or zero correlation. The key to determining and understanding a relation between variables is to identify the unit of measurement for each variable. Thus, be critical of a report which describes a correlation between variables but fails to define the unit of measurement of those variables unless it is obvious and commonplace.
Psychologists usually study behavior.
Often this involves verbal reports . There are advantages to using data based on what people remember, think, etc., but also serious problems. This raises the issue of reductionism . Further discussion on measurement, variables, and operational definitions is necessary, however. The author uses the example of hunger as an intervening variable.