How should I calculate quartiles? In a book I read (by Amir D. Aczel) it is said that ________. First quartile is the 25th percentile, that means a value for which 1/4 of observations' results are lower. Shouldn't it be lower or equal? According to the book if I have observations like this: 1, 3, 7, 9, 10, 11, the value of the first quartile is on position: (6+1) * 0.25 = 1.75, so it's 1.75. But shouldn't it be 3? For both 1.75 and 3, 25