I'm not sure if cognitive and psychomotor skill development can be directly compared, so I wouldn't use a tennis analogy when looking at homework. As for time on task, how much more than 6 hours of "book learning" can a child do in a day? Children need energy and concentration to do "effortful study", and this is difficult after 6 hours in school and then starting homework in the late afternoon or early evening. As author Sara Bennett says, "For many kids, homework is like having to do their taxes every night" - yuk.

Another difficulty in studying this phenomenon is the validity of the test scores themselves. Even if there was a positive correlation (which there is not) between homework and test scores; so what? Do higher test scores make for a better society? Do students with higher test scores found start-up companies and create wealth for their communities? Do they become social activists and help make this a better world?

There is more to life than school and there is more to learning than doing homework. Six hours a day, ten months per year, over 12 years, is enough time for teaching and instruction.