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A one-group posttest only design is a quasi-experimental approach where a researcher applies a treatment or manipulates an independent variable, and then measures the dependent variable just once after the intervention. It is considered the weakest quasi-experimental design because it lacks a control or comparison group. Without this comparison, there is no way to determine what the participants' outcomes would have been had they not completed the treatment.

One-Group Posttest Only Design

A practical example of a one-group posttest only design is measuring elementary school students' attitudes toward illegal drugs immediately after they complete an anti-drug education program, without any prior measurement or control group. Another common example involves media reports, such as an advertiser claiming that $$80\%$$ of women noticed brighter skin after using a specific cleanser for a month. In both cases, the lack of a comparison group makes it impossible to draw meaningful conclusions about the actual effectiveness of the treatment.

Examples of One-Group Posttest Only Designs

A one-group pretest-posttest design is a quasi-experimental research method where a single group of participants is measured on a dependent variable both before and after a treatment is administered. Unlike a within-subjects experiment, this design does not counterbalance the order of conditions because the treatment must inherently occur between the initial baseline measurement (pretest) and the subsequent measurement (posttest). While an improvement in posttest scores compared to pretest scores might intuitively suggest that the treatment was effective, such conclusions are difficult to make with certainty because the absence of a control group leaves the study highly vulnerable to alternative explanations and threats to internal validity.

One-Group Pretest-Posttest Design

Why is the one-group posttest only design considered the weakest quasi-experimental approach?

If a researcher implements a new study technique with a class of students and only measures their performance at the end of the semester, they are using a one-group posttest only design and can confidently establish that the new technique caused the students' final grades.

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