Research Methods and Statistics

CHAPTER PREVIEW

In the previous chapter we provided an overview of probability theory and statistical hypothesis testing. We can use statistical tests when we conduct experimental studies. Statistical tests are one part of applying experimental methods that allow researchers to establish a cause-andeffect relationship between an independent variable and a dependent variable. Experimental designs employ random assignment, which allows us to establish cause-and-effect relationships. Random assignment ensures that each person has an equal chance of ending up in a group on the basis of an objective placement. In the simplest experiment, the researcher compares a treated group (experimental) to a control (nontreated) group. This simple two-group design, or independent measures design, allows the researcher to compare two different levels of the independent variable using two different groups of people.

Instead of using two different groups, we can also design an experiment using only one group and ask participants to engage in both the treatment and control conditions of the study. In this case, we are still testing two levels of the independent variable, but we are using one group of people and asking them to participate in both conditions. The repeated measures design, using one group of people in both conditions, allows us to test the effect of the two levels of the independent variable using only one group of people. In each of these instances we are using two levels of a single independent variable, and we are measuring a single dependent variable. In the first case, we are using an independent samples design or a two-group design, and in the second instance we are using a related-samples or repeated measures design.

In many research studies, we use several different levels of the independent variable. For example, we might test the effect of three different amounts of caffeine. The simplest design employs only two levels of the independent variable. In this chapter, we will describe how to use inferential statistics to test differences between only two levels of the independent variable. The independent samples t test is used to test differences between two groups. The related samples or repeated measures t test is used to detect differences when one group of participants experiences both levels of treatment.