The Statistics range rule of thumb is used to decide if a value is unusual. The easy way for me to remember the rule of thumb formula is by the distance. If the distance is greater than or equal to 2 standard deviations away from the mean the event is unusual.
If a number seems high or low do the test if it is a discrete probability distributions or binomial probability distributions problem. The probability of an unusual event is between 5% to 0(repeating)1%.February 18, 2013 by Joseph Schumacher