Weighted average
The straight average for the morning class is 80 and the straight average of the afternoon class is 90. The straight average of 80 and 90 is 85, the mean of the two class means. But this does not account for the difference in number of students in each class, so the value of 85 does not reflect the average student grade (independent of class). The average student grade can be calculated by averaging all the grades, without regard to classes (add all the grades up and divide by the total number of students):