Two standard deviations below the mean is used to define an outlier.

• A. Sometimes
• B. True
• C. False
• D. Not used

Answer: (B)

Outliers are data points that lie far from the center of the distribution. A common rule-of-thumb is to flag observations that fall more than two standard deviations below the mean as outliers on the lower end (and similarly, more than two standard deviations above the mean as high outliers). This is because, under a roughly normal distribution, about 95% of values fall within two standard deviations of the mean, making those beyond that range unusually small or large. So, using two standard deviations below the mean to define an outlier is a widely used criteria, though some contexts may use three standard deviations or IQR-based methods instead.