Answer: 3σ.
- A σ
- B 3σ
- C 9σ
- D σ/3
Correct answer: B. 3σ
Explanation: Scaling each observation by 3 scales the standard deviation by |3|, giving 3σ.
In a perfectly normal (bell-shaped) distribution, mean, median, and mode all coincide at the centre; about 68% of data falls within 1 standard deviation of the mean, 95% within 2, and 99.7% within 3 - the empirical rule used to judge how typical or extreme a value is.
Concept context
Mean, median, mode, standard deviation, and data interpretation