The variance of a data set is the average squared distance between the mean of the data set and each data
value. An example of what this means is the graph that represents the results of 100 tosses of a fair coin, which resulted in 45 heads and 55 tails. The mean of the results is 50. The squared distance between the heads value and the mean is (45 - 50)^2 = 25 and the squared distance between the tails value and the mean is (55 - 50)^2 = 25. The average of these two squared distances gives the variance, which is 1/2 (25 + 25) = 25.