A group of researchers was interested in how group size affected people's ability to notice that an emergency situation was occurring. In this study, a participant completed a questionnaire in a waiting room either alone, with two confederates (people who were in on the experiment), or with two other naive participants. Shortly after starting the questionnaire, smoke was pumped into the room through a vent. The confederates ignored the smoke and waited for the participants to react. The time it took the participant to react to the smoke was measured by an experimenter (reported below in seconds).

What is the independent variable and how many levels does it have?