In psychology lab, Tetiana is conducting an experiment on depth perception using a box with two vertical rods and a ruler. The participant moves the rods until they appear to be aligned at the same distance, and then the experimenter measures how far out of alignment they are. Participants complete the task first with only their left eye open, then with only their right eye open, and then with both eyes open. Tetiana is using a repeated-measures design. Required:
How is the independent variable being manipulated in Tetiana's design?