You perform an experiment in which you take 16 pots of strawberry plants and give half of them 1 gram of ammonium nitrate per liter of water and the other half just gets water. Each group is then split in half again, and exposed to either 8 or 16 hours of light each day. You monitor the height of the plants for four weeks. You observe that increasing ammonium nitrate and light both increase plant height. In this experiment, you are working with available planter pots from the laboratory. You have choices of clay pots of different sizes, as well as biodegradable peat pots and others within lab equipment provided. To establish a standardized variable, you would :