Measures of the Centre: The arithmetic mean, median,
Measures of Variability: The range, variance, standard
CHOOSE ANY NUMBER k GREATER THAN 1
General case: AT LEAST 1 - 1/k of numbers are less
than k standard deviations away from the mean.
k = 2: AT LEAST 3/4 of the numbers are less than 2
standard deviations away from the mean.