An observation from a normally distributed population is considered "Unusual" if it is more than 2 standard deviations away from the mean.

An observation from a normally distributed population is considered "Unusual" if it is more than 2 standard deviations away from the mean. There are several contaminants that can harm a city's water supply. Nitrate concentrations above 10 ppm (parts per million) are considered a health risk for infants less than six month of age