Students must give consent before starting and can withdraw from a study at any time without penalty. If you have clearly marked your requirements, and students show up who do not fit the requirements, you do not need to award them points.
Student No Shows
Some semesters the no show rate for some studies is high, which is frustrating to researchers. Researchers naturally want to punish these slackers somehow, but several years ago, the National IRB ruled that not showing up for a study was equivalent to choosing to withdraw from a study--the research participant's right--so no penalties can be administered.
Researcher No Shows
If students show up for a study and the researcher does not, the student should be awarded full points as if he or she had participated. They do NOT need to complete the study at another time. A student can earn double points for your study—once for completing it, once if you or your researcher fails to show up. Students should email the researcher if they showed up for an event and the researcher was a no show so please respond to student emails. Students should also be given their points if they show up and the equipment doesn't work or some other researcher issue arises. If the error is on the researcher's side, the participant who shows up should be awarded points.