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In statistics, the 68–95–99.7 rule, also known as the empirical rule, and sometimes abbreviated 3sr, is a shorthand used to remember the percentage of values that lie within an interval estimate in a normal distribution: approximately 68%, 95%, and 99.7% of the values lie within one, two, and three standard deviations of the mean, respectively.
A pie chart showing the percentage by web browser visiting Wikimedia sites (April 2009 to 2012) In mathematics, a percentage (from Latin per centum 'by a hundred') is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign (%), [1] although the abbreviations pct., pct, and sometimes pc are also used. [2]
Thus, rounding to two decimal places, −3σ is the 0.13th percentile, −2σ the 2.28th percentile, −1σ the 15.87th percentile, 0σ the 50th percentile (both the mean and median of the distribution), +1σ the 84.13th percentile, +2σ the 97.72nd percentile, and +3σ the 99.87th percentile.
The top grade, A, is given here for performance that exceeds the mean by more than 1.5 standard deviations, a B for performance between 0.5 and 1.5 standard deviations above the mean, and so on. [17] Regardless of the absolute performance of the students, the best score in the group receives a top grade and the worst score receives a failing grade.
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[13] [14] This is casually referred to as "three and a half nines", [15] but this is incorrect: a 5 is only a factor of 2, while a 9 is a factor of 10, so a 5 is 0.3 nines (per below formula: ): [note 2] 99.95% availability is 3.3 nines, not 3.5 nines. [16]
The median 401(k) balance for the same participants — the middle number when you line up all balances from lowest to highest — paints a different picture at just $35,286, almost $100,000 less ...