Rule of Thumb.

Date: 2015-10-07; view: 357.

When a distribution is bell-shaped the following statements, which are called Thumb rule, are true:

Approximately 68% of the population members lie within one standard deviation of the mean.

Approximately 95% of the population members lie within two standard deviations of the mean.

Approximately 99.7% of the population members lie within three standard deviations of the mean.

For example, suppose that scores on entrance exam have a mean of 480 and standard deviation of 90. If these scores are normally distributed, then approximately 68% will fall between 390 and 570 ;

and

Approximately 95% of the scores will fall between 300 and 660

and .

Approximately 99.7 % of the scores will fall between 210 and 750

and .