(add up all the data set and divide it by 7, why 7?, because there are about 7 numbers in the data set or the listed numbers)
2 + 3 + 5 + 7 + 8 + 8 + 9 = 49
------ = 7
7
(subtract the data set and 7)
7 - 2 = 5
7 - 3 = 4
7 - 5 = 2
7 - 7 = 0
7 - 8 = 1
7 - 8 = 1
7 - 9 = 2
(add the difference and divide it by 7 again)
5 + 4 + 2 + 0 + 1 + 1 + 2 = 15
------- = 2.14
7
final answer would be 2.14