Basic Study Designs

There are two basic types of study designs: Descriptive and Evaluative. The goal of descriptive studies is to describe some phenomenon based on observations gathered in various ways. Types of descriptives studies include case studies, observation, interviews, focus groups, or surveys. Another module covers these types of studies in more detail.

The goal of evaluative studies is to test a hypothesis or theory based upon on observations. Types of evaluative studies included controlled experiments, quasi-experiments, or mixed methods. The following text describes four basic types of evaluative studies.

Single Group Post-Test Only

This is the most basic type of study. As the name suggests, this study design has only one group, meaning that all participants undergo exactly the same procedure. This study design also gathers data using only a post-test, or a data collection activity performed at the end of the study. As the figure below shows, there are two primary steps in this study design:

• Introduce the Independent Variable (Treatment)
• Measure the effect (or gather the Dependent Variable)

The biggest weaknesses to this approach are:

• There is no comparison group to help the researcher understand whether the measure of the Dependent Variable is good.
• There is no baseline or measure before the treatment to allow the researcher to observe change.

These weaknesses prompt the development of more complicated study designs.

Single Group Pre-Test/Post-Test

In this study design, the researcher addresses one of the weaknesses from the Single Group Post-Test Only design by adding a pretest. In this case, the researcher gathers some type of baseline measurement prior to introducing the independent variable. This pre-tests allows the researcher to observe whether there is any change in the participants after introducing the independent variable. The figure below illustrates this change by adding the pre-test step before the introduction of the independent variable (treatment).

When should a researcher consider using a pre-test?

• When there is a small sample size
• When they want to measure the changes in individual participants
• To understand the effects of mortality, that is participants who do not complete the study.

What are the disadvantages of using a pre-test?

• It can be time-consuming to add another step in the process
• It can affect behavior. If somehow completing the pre-test changes the participants (for example they learn something they did not know before), then the pre-test could change the results.

Two Groups Post-Test Only

In this study design, the researcher addresses another of the weaknesses from the Single Group Post-Test Only design by adding a control group. In this case, the researcher is able to gain some insight into whether the independent variable has an effect by observing another group of participants who do not receive the application of the independent variable. As the figure below shows, the researcher divides the participants into two groups. One group receives the independent variable and the other does not. Then the researcher measures the dependent variable on participants from both groups.

While this design improves over the Single Group Post-Test Only design, it still lacks a pre-test that can provide a baseline.

Two Groups Pre-Test/Post-Test

In this study design, the researcher addresses both of the weaknesses from the Single Group Post-Test Only design by adding both a pretest and a control group. It therefore has the benefits described in the two study types above. The figure below illustrates this design.

This type of study can help in cases where the participants may change over time and the researcher wants to understand whether application of the independent variable affects the rate of change. For example, in a classroom, students are hopefully learning more about the course topic as the semester progresses.

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