A good rule of thumb (though not a hard-and-fast rule) is that any value that is smaller than _____ standard deviations below the mean (or _____ standard deviations above the mean), is a statistical outlier.

• A. One
• B. Two
• C. Three
• D. Four

Answer: (B)

Outliers are values that sit far from the center of the data. A practical rule uses the spread of the data, measured by the standard deviation. In roughly normal data, about 95% of observations lie within two standard deviations of the mean, so anything beyond mean minus two standard deviations or beyond mean plus two standard deviations is considered unusual and labeled as an outlier. This is a widely used rule of thumb, not a strict rule, because real data can be skewed or have heavy tails. For those cases, or with small samples, alternatives like the IQR-based method can be more appropriate. So the blanks are filled with two standard deviations in each direction.