An experimenter conducts a study in which she wants to examine the effects of altitude on psychological well-being. To do this, she randomly allocates people to two groups. She takes one group up in a plane to a height of 1000 meters and leaves the other group in the airport terminal as a control group. When the plane is in the air, she seeks to establish the psychological well-being of both groups.



Which of the following is a potential confounding variable?


The reliability of the questionnaire that she uses to establish psychological health


The size of the space in which the participants are confined


The susceptibility of the experimental group to altitude sickness


The susceptibility of the control group to altitude sickness