Piliavin et al. (1969) implemented strict standardisation in their 'Subway Samaritan' field experiment to maintain internal validity. However, because the study was conducted in a natural setting on a live New York transit line, several extraneous variables remained uncontrolled. Which of the following represents an uncontrolled environmental or participant variable that threatened the internal validity of their study?
The specific time interval of 70 seconds70\text{ seconds}70 seconds after departure at which the victim collapsed.
The physical appearance, dress, and standardized posture of the victim across all trials.
The potential for commuters to experience repeated exposure to different trials on the same transit route.
The specific subway line, carriage layout, and location of the collapse next to the metal pole.
