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.