Stephanie M. answered • 05/19/15

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The 68-95-99.7 Rule states that 68% of a normal distribution's values are within one standard deviation of the mean. 95% are within two standard deviations and 99.7% are within three standard deviations.

http://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule

That means that the proportion of values within one standard deviation is 68/100 = 17/25.

David W.

The 68-95-99.7 rule is such a rule. It allows you to calculate percentage (proportion) of the normal frequency distribution that is within 1, 2,or 3 standard deviations. Later, in calculus, you can take the rather complicated formula for the normal curve and calculate the area under (what’s called integrate) it to determine what percentage of the total area (or percent of the observations in the population) are within that range.

Just as an observation (pun intended), proportions of the normal distribution are very likely (also a probability and statistics term) to be represented as a percent (here, 68%).

05/19/15