The percent of cases that are at or below a score.
But we only count half of the cases that are at the score.
Example: in a recent test 12% got D, 50% got C, 30% got B and 8% got A
You got a B, so add up:
• all the 12% that got D,
• all the 50% that got C,
• half of the 30% that got B,
for a total percentile rank of 12%+50%+15% = 77%
In other words you did "as well or better than 77% of the class"
(Why take only half of B? Because you shouldn't imagine you got the "Best B", or the "Worst B", just an average B)
