Hoping to increase the average number of tomatoes produced per plant, Terrie plans an experiment.
She randomly selects 16 Tycoon tomato plants and 16 Park's Whopper tomato plants from the local nursery. Each group of 16 is then randomly split into 4 sets (A, B, C, D) of 4 plants.
Set A receives a high-nitrogen fertilizer and full sun.
Set B receives a low-nitrogen fertilizer and full sun.
Set C receives a high-nitrogen fertilizer and part shade.
Set D receives a low-nitrogen fertilizer and part shade.
At the end of the growing season, Terrie compares the number of tomatoes produced from the four sets within each variety group.
The variable being blocked in this experiment is