one of the 100 equal groups that a larger group of people can be divided into, according to their place on a scale measuring a particular value百分位数
Overall these students rank in the 21st percentile on the tests—that is, they did worse than 79 per cent of all children taking the test.这些考生的总体百分位排名占第 21 位。就是说,79% 的应试儿童比他们考得好。