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Nursing 472: Statistics


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68 95 99.7 Rule

In a normal distribution, data is distributed in a perfect bell curve.  The mean is the middle line on the diagram.  Each vertical line on either side of the mean represents one standard deviation.  68% of the data lies within one standard deviation of the mean (this mean one standard deviation on both sides of the mean).  95% of data lies within 2 standard deviations of the mean.  99.7% of data lies within 3 standard deviations of the mean. 

Example 1: Waiting time in the ER.  The mean waiting time is 60 minutes.  The standard deviation is 10 minutes.  If the data has normal distribution, how long do 68% of people wait?

Answer to Example 1: 68% of people wait between 50 and 70 minutes.  In this question, you both add and subtract 10 (one standard deviation) from the mean to get the answer. 

*Helpful Hint: If you wanted to know how long 95% of people wait, you would add and subtract 20 minutes (2 standard deviations) from 60 (the mean) to get the answer 40-80 minutes. 

Example 2: How long do 99.7% of people wait?

Answer to Example 2: 99.7% of people wait between 30 and 90 minutes.  To get this answer, you both add and subtract 30 (3 standard deviations) from 60 (the mean).


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Guide developed with assistance from S. Pruitt, student, SIUE School of Nursing, and 2014 URCA program assistant.