Measure knowledge pre and post online course using Assessments

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Hello,

In 18B we can use Assessments to measure knowledge. I'm trying to find a way on how to manage assessments in order to measure learners knowledge before and after the course completion. If I put the assessment as an activity under the same offering with the self-paced activity the learner can take the assessment before completing the course. How can I restrict the learner when they can take the assessment? I don't want to manage different course->offering.

Thanks,

Daniela