500
These are two things that I would need to keep as constants if I was designing an experiment in the following way:
I'm going to test what beverage is best at keeping me awake in the morning. I'm going to test coffee the first week, soda the next week, and water the final week.
What are: Amount of each beverage consumed, time in the morning that the liquid is drank, amount of sleep I get the night before, the breakfast that I eat, (others acceptable)?