Dr. Lee is interested in studying how different lighting conditions affect people's productivity at work. A sample of 100 office workers is selected, and their productivity is measured under standard lighting conditions for several days to get baseline readings. Then, 50 workers are randomly assigned to the bright lighting condition, where the office lights are set to a higher intensity. The other 50 workers are assigned to the dim lighting condition, where the office lights are set to a lower intensity. For several days, Dr. Lee collects data on the workers' productivity to see how the different lighting conditions have impacted their performance. In this example, the productivity of the workers is the