Answer
When the probability distribution is approximately bell shaped. And, given that the distribution is bell shaped, approximately 95% of all observations will be between $μ-2σ$ and $μ+2σ$. So, if a result is more than 2 standard deviations of the mean or less than 2 standard deviations of the mean, it is an unusual event.
Work Step by Step
We must have $np(1-p)\geq10$ in order for the probability distribution to become approximately bell shaped.
