Arithmetic mean

1. The mean length of sentence is the total of the values divided by the total number of values:

(14 + 16 + 18 + 15 + 16 + 17 + 16) / 7 = 16 months

2. Adding in a five year sentence (60 months), the calculation now becomes:

(14 + 16 + 18 + 15 + 16 + 17 + 16 + 60) / 8 = 21.5 months

This does not really reflect the distribution of data. One extremely high result has distorted the mean.