Question

A statistics practitioner determined that the mean and standard deviation of a data set were 120 and 30, is respectively. What can you say about the proportions of observations that lie between each of the follow- ing intervals?

a. 90 and 150
b. 60 and 180
c. 30 and 210

Answer 1

a) 68% of the observations lie between 90 and 150.

b) 95% of the observations lie between 60 and 180.

c) 99.7% of the observations lie between 30 and 210.

Explanation:

The Empirical Rule states that, for a normally distributed random variable:

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

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

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

In this problem, we have that:

Standard deviation = 120

Mean = 30

a. 90 and 150

This is within 1 standard deviation of the mean.

By the Empirical Rule, 68% of the observations lie between 90 and 150.

b. 60 and 180

This is within 2 standard deviations of the mean.

By the Empirical Rule, 95% of the observations lie between 60 and 180.

c. 30 and 210

This is within 3 standard deviations of the mean.

By the Empirical Rule, 99.7% of the observations lie between 30 and 210.