Microsoft Office Tutorials and References
In Depth Information
Deviating from the Middle
Figure 9-6 takes the data and variance shown in Figure 9-4 and adds the
standard deviation to the picture. The standard deviation is 3.762977544. This
number fits inside the range of the sample data.
Figure 9-6:
Calculating
the standard
deviation.
Looking for normal distribution
The standard deviation is one of the most widely used measures in statistical
work. It’s often used to analyze deviation in a normal distribution. A
distribution is the frequency of occurrences of values in a population or sample.
A normal distribution often occurs in large data sets that have a natural, or
random, attribute. For example, taking a measurement of the height of 1,000
10-year-old children will produce a normal distribution. Most of the
measured heights will center around and deviate somewhat from the mean. A few
measured heights will be extreme — both considerably larger than the mean,
and considerably smaller than the mean.
Ringing the bell curve
A normal distribution is often visually represented as a graph in the shape of
a bell — hence the popular name, the bell curve. Figure 9-7 shows a normal
distribution.
A normal distribution has a few key characteristics:
The curve is symmetrical around the mean — half the measurements
are greater than the mean and half are less than the mean.
The mean, median, and mode are all the same.
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