In a normal curve, what percentage is within 1 standard deviation of the mean?

• A. 50%
• B. 95%
• C. 99.5%
• D. 68%

Answer: (D)

In a normal distribution, most observations cluster around the mean and the spread is set by the standard deviation. The empirical rule describes how much of the data falls within certain distances from the mean. The interval that stretches from one standard deviation below the mean to one standard deviation above the mean contains about 68% of the observations. More precisely, it’s about 68.27%, which is usually rounded to 68%. So this central band around the mean includes roughly two-thirds of the data, which is why 68% is the correct answer. If you look further out, about 95% fall within two standard deviations, and about 99.7% fall within three standard deviations. The 50% option doesn’t correspond to the standard normal distribution intervals.