"if the difference between the 20th and 21st best players is less than the measurement error, you've still done that optimally." I thought that the coach was r

"if the difference between the 20th and 21st best players is less than the measurement error, you've still done that optimally." I thought that the coach was responsible for minimizing the measurement error. Then, the best he could do is to pick the best team. How do you define measurement?