Types of Quasi-experimental Designs

Notes taken from Jackson, S.L. (2006). Research Methods and Statistics: A Critical Thinking Approach. Belmont, CA: Thomson Wadsworth

1. Single-Group Posttest-Only Design

• A single group of participants is given a treatment and then tested. 
• Problem: We cannot claim a method (treatment) is better when we cannot compare the results for the group who participated with the results for any other group or standard. 
• Cannot be used to draw conclusions about how an experience has affected the individuals involved.

2. Single-Group Pretest/Posttest Design

• A single group of participants takes a pretest, then receives some treatment, and then takes a posttest measure. 
• Any differences in the measures are assumed to be the result of the treatment.
• Problem: The lack of a comparison group, i.e. we do not know whether any observed change is due to the treatment or something else that may have happened during the time of the study.  

3. Single-Group Time-Series Design

• A single group of participants is measured repeatedly before and after a treatment. 
• Advantage: the multiple measures allow us to see whether the behavior is stable before treatment and how, or if, it changes at the multiple points in time at which measures are taken after treatment. 

4. Nonequivalent Control Group Posttest-Only Design

• At least two nonequivalent groups are given a treatment and then a posttest measure.
• The control group is nonequivalent, meaning that participants are not assigned to either the experimental or the control group in a random manner. Instead, they are members of each group because of something that they chose or did--they come to the study already a member of one of the groups. 
• There is no assurance that the two groups are at all equivalent on any variable prior to the study. 
• Thus, we cannot say definitely that the treatment is responsible for any observed changes in the groups. It could be that the groups were not equivalent at the beginning of the study; hence, the differences observed between the two groups on the dependent variable may be due to the non-equivalence of the groups and not to the treatment.

5. Nonequivalent Control Group Pretest/Posttest Design

• At least two nonequivalent groups are given a pretest, then a treatment, and then a posttest measure. 
• A pretest allows us to assess whether the groups are equivalent on the dependent measure before the treatment is given to the experimental group. 
• We can assess any changes that may have occured in each group after treatment by comparing the pretest measures for each group with their posttest measures.
• So, we can compare performance within each group from the pretest to the posttest. 
• If the treatment had some effect, then there should be a greater change from pretest to posttest for the experimental group than for the control group. 

6. Multiple-Group Time-Series Design
A series of measures are taken on two or more groups both before and after a treatment.