The principal of an elementary school is trying to do a performance analysis of the 5th-grade test grading process. There are 6 homeroom classes in 5th grade. Each class has 30 students. Each student faces 3 exams during the school year (assume that there were no absences for exams). It was found that 4 tests were graded incorrectly during the school year. 

a) What is the tempo metric for the test grading process? 

b) What sigma level does it correspond to? 

(You may need to use the Excel template)