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Standard Deviation

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Submitted By groovylue
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Standard Deviation (1 of 3)

Introduction
So far, we have introduced two measures of spread; the range (covered by all the data) and the inter-quartile range (IQR), which looks at the range covered by the middle 50% of the distribution. We also noted that the IQR should be paired as a measure of spread with the median as a measure of center. We now move on to another measure of spread, the standard deviation, which quantifies the spread of a distribution in a completely different way.

Idea
The idea behind the standard deviation is to quantify the spread of a distribution by measuring how far the observations are from their mean, x. The standard deviation gives the average (or typical distance) between a data point and the mean, x.

Notation
There are many notations for the standard deviation: SD, s, Sd, StDev. Here, we'll use SD as an abbreviation for standard deviation, and use s as the symbol.

Calculation
In order to get a better understanding of the standard deviation, it would be useful to see an example of how it is calculated. In practice, we will use a computer to do the calculation.

Example: Video Store Customers

The following are the number of customers who entered a video store in 8 consecutive hours:
7, 9, 5, 13, 3, 11, 15, 9

To find the standard deviation of the number of hourly customers: 1. Find the mean, x



of your data: (7+9+5+. . .+9) 
 = 9 8 2. Find the deviations from the mean: the difference between each observation and the mean (7 - 9), (9 - 9), (5 - 9), (13 - 9), (3 - 9), (11 - 9), (15 - 9), (9 - 9)


 -2, 0, -4, 4, -6, 2, 6, 0 Since the standard deviation is the average (typical) distance between the data points and their mean, it would make sense to average the deviations we got. Note, however, that the sum of the deviations from the mean, x
⎯
⎯


is 0 (add them up

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