Question

Your teacher is doing and long term experiment by having you and your classmates grow plants at home. You are testing the hypothesis that plant growth is affected by the amount of water a plant receives. All the data will be compiled in three weeks. Why isn't it A good idea to use the 8 ounce measuring cup from your kitchen or the 12 inch ruler you have on your desk ?

Answer 1

Because ounce and inch system are too big thus less sensitive. 
Using metric systems like milliliter and millimeter might be better since you can find smaller differences. If you are using a system that less sensitive you can observe small differences and might need to grow the trees bigger than it should.