Just don't sign-up for the same experiment number twice. Sometimes experimenters will have different sessions of the same experiment number on different days and times. You won't get credit if you show up for an experiment you've already done. You must be on time for experiments, or they may start without you. If that happens, or if you miss an appointment entirely, you'll be counted as a no-show.
Experiments begin on the hour or half-hour only.
Some of the longer experiments may consist of more than one session. Please remember that you'll be going back for another session and don't sign up for additional experiment sessions. If you are in a multiple session experiment, you should be given credit for the work completed at the end of each session. You may remove your name from an experiment on the web up to 24 hours before the experiment. Or, if the experimenter has called you, you can call the experimenter up to 24 hours before the experiment time to cancel.