**Many Labs 3: Participant Pool Edition** **Empirically evaluating pervasive beliefs about participant pool quality across the academic semester** **Overview** Many psychologists rely on undergraduate participant pools as their primary source of participants. Most participant pools are made up of undergraduate students taking introductory psychology courses over the course of a semester. Each semester the pool refreshes with a new group of students. In most, students self-regulate when to participate – either for extra credit or to meet a course requirement. Because of this self-selection, the qualities of participants may vary across the semester - influencing both the sample characteristics and the likelihood of detecting effects. This issue is relevant to all behavioral researchers that use university participant pool. As such, there are pervasive superstitions, lay theories, and anecdotal examples about when is the best time to collect data for a particular effect. For example, some never collect data at the end of the semester; others only do so. Remarkably, there is little to no systematic evidence for whether the timing of pool data collection affects the power and sensitivity of experimental designs. Part of the reason is the feasibility of obtaining sufficient data to test this question. Many Labs 3 is a crowdsourced project that will systematically evaluate time-of-semester effects across many participant pools. The project team created a single experimental protocol assessing 10 known effects within 30 minutes. Twenty three labs will administer the same protocol across the academic semester. The aggregate data will provide high-powered evidence whether time-of-semester moderates the detectability of effects. Effects will be selected based on the lay theories of what is likely to be influenced by time of semester. **Files** Final protocol, effect description, and analysis plans: [ML3_Protocol_9152014.pdf][1]) Experimenter script: [ML3_Computer_Scripts_by_collection_site.zip][2] Video demo of procedure: [UVa_ML3_Demo.MOV.zip][3] Scripts to administer the experimental procedure: [ML3_Lab_Script.docx][4] Packets of stimuli and questions for in-lab portion: [ML3_In-Lab_Packetsrevised.docx][6] Summary of effects and individual difference measures: [ML3: Selected Effects and Individual Difference Measures][5] [1]: https://osf.io/wamcn/ [2]: https://osf.io/vsxkg/ [3]: https://osf.io/w358c/ [4]: https://osf.io/ud3na/ [5]: https://docs.google.com/spreadsheets/d/1db7KIJws0ttGc3vlD5WxwXsvK61o2o67JMIzzP1aaqk/edit?usp=sharing [6]: https://osf.io/2zatr/