<p>As an engineer, I am painfully aware of the law of diminishing returns. Note that I said that going from two to four hours of practice would show more results than going from five to seven. </p>
<p>Someone with 18 years of lessons under their belt could improve dramatically after a year spent with a new and much better teacher. Diminishing returns can take a while to reappear once something important in the process has been changed, as often happens when a student progresses from high school to college or from undergraduate to graduate work. If an extra year were not going to be beneficial at that point, then there would be a lot fewer performance majors in graduate school.</p>