In a field experiment investigating helping behavior, researchers Nicolas Guéguen and Marie-Agnès de Gail tested whether being smiled at increased the likelihood that supermarket shoppers would help a confederate pick up dropped computer diskettes. Because this was a naturalistic setting without formal recruitment, the researchers utilized strict, predetermined selection rules to prevent unintentional experimenter bias when choosing participants. Specifically, they targeted the first person appearing to be between the ages of $20$ and $50$ encountered on a stairway, and only if the person gazed back did they become a participant in the study.