This week have been experimenting with a trebuchet.
We think that if you put more weights in the counter weight, the gravity will make the tennis balls go farther because gravity pushes the counterweight down and makes the weight pull the pouch up and make the ball fly out.
When we put a heavier ball in the pouch and fired, it it didn't go as far, because to make it go as far as the lighter ball it will need more weight.
My data gathering is unistructural because I made one observation. My inferences are good, I am confident in how I set up the catapult.