Unusual results are those that have a very low chance of occurring. Unusual results can be identified using probabilities and the range rule of thumb. In problems involving probability, unusual results can be observed in 2 instances – an unusually high number of successes or an unusually low number of successes.

According to the range rule of thumb, any value above or below two standard deviations, 2σ from the mean, μ is considered unusual.

Maximum unusual value = μ + 2σ

Minimum unusual value = μ - 2σ