Final answer:
Joe should say that there is a correlation between height and serve speed; however, there is no causation.
Step-by-step explanation:
Based on Joe's findings, he should say that there is a correlation between height and serve speed; however, there is no causation. This is because there is an increase in serve speed with an increase in height. Joe's comparison of player statistics from his team's tennis season shows that taller players tend to have faster serves. However, it is important to note that correlation does not necessarily imply causation, so further studies would have to be done to determine the exact relationship between height and serve speed.