What about variability and stride? A teacher's performance is likely to vary daily, as well between years. How do we know, rejecting a mediocre teacher, that

What about variability and stride? A teacher's performance is likely to vary daily, as well between years. How do we know, rejecting a mediocre teacher, that she wouldn't have blossomed into a superstar? How do we know that the trigonometry teacher won't become cynical with age? The second problem can be solved by gathering data every few years, but what about the first?