Quartiles

The quartiles are three values which divide the sorted data set into four (approximately) equal parts. We cannot divide a set of 17 (say) data values into four equal parts since that would require splitting a datum. What we can do is interpolate between data items which bracket the desired position.

     i    x[i]
     1    102
     2    105
    ----------- the first quartile, Q[1] = (105+106)/2 = 105.5
     3    106
     4    109
   ------------ the second quartile or median or Q[2] = 109.5
     5    110
     6    112
   ------------ the third quartile, Q[3] = (112+115)/2 = 113.5
     7    115
     8    118

back to summary statistics -- quantiles