What percent of a normal population is encompassed by a reference range defined as Mean ±2SD?

What percent of a normal population is encompassed by a reference range defined as Mean ±2SD?

• A. 68%
• B. 95%
• C. 99%
• D. 50%

Answer: (B)

In a normal distribution, the portion of data within a certain distance from the mean is described by the empirical rule. About 68% lie within one standard deviation of the mean, about 95% lie within two standard deviations, and about 99.7% lie within three standard deviations. Therefore a reference range defined as Mean ± 2SD encompasses roughly 95% of the population. That makes 95% the correct choice. The other options don’t fit: within ±1 SD is about 68%; needing about ±2.58 SD would capture ~99%; and 50% would be a narrower interval than ±2 SD.

In a normal distribution, the portion of data within a certain distance from the mean is described by the empirical rule. About 68% lie within one standard deviation of the mean, about 95% lie within two standard deviations, and about 99.7% lie within three standard deviations. Therefore a reference range defined as Mean ± 2SD encompasses roughly 95% of the population. That makes 95% the correct choice. The other options don’t fit: within ±1 SD is about 68%; needing about ±2.58 SD would capture ~99%; and 50% would be a narrower interval than ±2 SD.