Question

Suppose that a histogram of a data set is approximately symmetric and "bell shaped". approximately what percent of the observations are within one standard deviation of the mean?

a. 99.7%
b. 50%
c. 68%
d. 95%

Answer 1

c. 68%

Explanation:

The 68-95-99.7 Rule states that, for normally distributed samples:

68% of the measures are within 1 standard deviation of the mean.

95% of the measures are within 2 standard deviations of the mean.

99.7% of the measures are within 3 standard deviations of the mean.

So

Approximately what percent of the observations are within one standard deviation of the mean?

c. 68%

Answer 2

The empirical rule tells you 68% of normally-distributed observations are within 1 standard deviation of the mean.

_____

This is a number you may want to memorize, along with the numbers for 2 (95%) and 3 (99.7%) standard deviations.