Any value smaller (larger) than ______ standard deviations below (above) the mean is considered inconsistent?

• A. one
• B. two
• C. three
• D. four

Answer: (B)

Think in terms of how far a value typically can wander from the mean. A common rule of thumb is that for data that roughly follows a normal distribution, about 95% of observations lie within two standard deviations of the mean. So values that fall beyond that range are not typical and are often labeled inconsistent or unusual.

Why this threshold fits well: inside two standard deviations you’re capturing the majority of normal variation, while beyond it you’d expect only a small, rare portion of data. Using one standard deviation would flag too many ordinary observations as inconsistent, and using three standard deviations would be too strict, reserving the label for only the extreme outliers. Four standard deviations would be even more restrictive.

Thus, the value two standard deviations below or above the mean is the standard threshold for calling something inconsistent.