At pretest students are generally motivated to engage testing with reasonable levels of effort, so pretest data can be gathered by testing entering students as early in the instructional program as possible, ideally in the first session if not prior to the first session.  Pretesting the entire group of entering students is optimal.  However, a representative sampling of individuals may be sufficient for aggregate pretest to posttest group comparison.  Because a less than representative sampling can result in misleading aggregate data, permitting individuals to self-select into or out of pretesting (or posttesting) is not recommended.  Contact Insight Assessment educational testing specialists to discuss alternative approaches to the timing and gathering of pretest data.