In Darley and Latané’s bystander experiment, the independent variable was the perceived number of witnesses, which researchers manipulated to create three distinct conditions: participants believed there were either one, two, or five other students present. To maintain experimental control, the researchers held extraneous variables constant by testing everyone in the exact same room and exposing them to the identical simulated emergency. They also randomly assigned participants to these conditions to ensure the groups were similar from the start, effectively controlling for extraneous participant variables.