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Thumbnail of data collection design model. Click to open. Click to open the data collection design model.

In the posttest-only design, data are collected only once from participants, immediately after they complete the program. The image at right illustrates what posttest-only data collection looks like in a preexperimental study.

The major drawback of any posttest-only study is that it does not provide a baseline to which postintervention results can be compared. To assess change, you must be able to compare post intervention data with a baseline measure.

However, a posttest-only design can be appropriate if you only need to know—or if it is only feasible to know—if participants have reached an identified outcome (for example, learned a new skill), rather than measure the degree of change. Also, if you have limited resources to spend on the evaluation or limited access to program participants, this could be the best or only option.

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