To mix a metaphor, all good serves are the same; all bad serves are bad in their own way. A model might work well for people who generally serve well, or for an individual person who serves badly in a particular way. But how do you make a model that works for many different types of people, and many different types of serves?

It would be great to see if the data that the Apple Watch generates is similar to what speed guns show. That would make a great experiment for a high schooler!