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One-Group Pretest–Posttest Design

A one-group pretest–posttest design is a type of research design that is most often utilized by behavioral researchers to determine the effect of a treatment or intervention on a given sample. This research design is characterized by two features. The first feature is the use of a single group of participants (i.e., a one-group design). This feature denotes that all participants are part of a single condition—meaning all participants are given the same treatments and assessments. The second feature is a linear ordering that requires the assessment of a dependent variable before and after a treatment is implemented (i.e., a pretest–posttest design). Within pretest–posttest research designs, the effect of a treatment is determined by calculating the difference between the first assessment of the dependent variable (i.e., the pretest) and the second assessment of the dependent variable (i.e., the posttest). The one-group pretest–posttest research design is illustrated in Figure 1. This entry discusses the design’s implementation in social sciences, examines threats to internal validity, and explains when and how to use the design.

Implementation in Social Sciences

The one-group pretest–posttest research design is mostly implemented by social scientists to evaluate the effectiveness of educational programs, the restructuring of social groups and organizations, or the implementation of behavioral interventions. A common example is curriculum or instructor assessments, as instructors frequently use the one-group pretest–posttest research design to assess their own effectiveness as instructors or the effectiveness of a given curriculum. To achieve this aim, instructors assess their students’ knowledge of a given topic or skill at performing a particular behavior at the beginning of the course (i.e., a pretest, O1). Then, these instructors devote their efforts over a period of time to teaching their students and assisting them in acquiring knowledge or skills that relate to the topic of the course (i.e., a treatment, X1). Finally, at the conclusion of the course, instructors again assess students’ knowledge or skills via exams, projects, performances, or exit interviews (i.e., a posttest, O2). The difference between students’ knowledge or skills at the beginning of the course compared with the end of the course is often attributed to the education they were provided by the instructor. This scenario is commonly used within STEM disciplines (i.e., science, technology, engineering, and mathematics), but it is also utilized within the discipline of communication studies—especially within public speaking or introductory communication courses. This example will be referenced in the subsequent section to illustrate potential threats to the internal validity of the one-group pretest–posttest research design.

Threats to Internal Validity

The one-group pretest–posttest research design does not account for many confounding variables that may threaten the internal validity of a study. In particular, this research design is susceptible to seven distinct threats to internal validity that may promote inaccurate conclusions regarding the effectiveness of a treatment or intervention.

History Effects

The first type of threat is history effects, which acknowledges that events or experiences outside the scope of a study may influence the changes in a dependent variable from pretest to posttest. The longer a research design takes to execute and the more time participants spend outside the controlled environment of an experiment or study, the greater the chance that the posttest can be influenced by unaccounted for variables or experiences. For instance, in the aforementioned example, students will spend a majority of their time outside of the confines of the classroom. As such, the growth in their knowledge or skills may be explained by experiences besides the few hours of instruction they receive per week (e.g., they could learn in other classes, watch documentaries on their own time, or refine their skills as part of extracurricular experiences).

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