During their field experiment on the New York subway, Piliavin et al. (1969) implemented strict standardization to maintain internal validity. Which of the following features of the study was successfully kept constant as a standardized control by the researchers?
The density of the passenger group in the critical and adjacent areas of the carriage
The spontaneous movement and verbal remarks of the bystanders in response to the collapse
The precise time elapsed after train departure before the victim collapsed
The demographic distribution (gender and race) of the passengers on the subway train
