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.
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
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.