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You can improve on the pre- and posttest design by adding follow-up measurement points—for example, at 3, 6, or 12 months postintervention, or perhaps earlier, depending on the likelihood of finding effects in the short and long term.
The pre- and posttest with follow-up design enables you to gain a more longitudinal (i.e., over time) perspective on your program. The image at right illustrates what data collection looks like in this type of design.
Follow-up measurements can tell you if a program has a sustained effect, yielding changes in behavior or knowledge that persist beyond the immediate treatment period. However, it is sometimes difficult and costly to locate participants for follow-up assessments, and you will need to plan carefully how you will ensure participant retention over the follow-up period.
When the same instrument is administered multiple times, the test repetition is also a potential confound. Participants could become bored or annoyed by having to complete the same measure again, or they may be less interested in participating months or years later. These situations can affect the response rate for follow-ups as well as the accuracy and validity of the responses that are obtained. However, these are issues that you may be able to control through planning and careful administration of your measures.
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