Microsoft Office Tutorials and References

In Depth Information

**Theories About Standard Deviation**

are a random sample, then variance is calculated by dividing DEVSQ

by 20
−
1, or 19. You use VAR.S in Excel to calculate this.

•
The measurement for variance is a square, right? You took all the

deviations, squared them, and then averaged (or nearly averaged)

them. The final popular measure of dispersion is calculated by taking

the square root of the variance. This number is called standard devi-

ation. Excel offers two functions for standard deviation. You use

STDEV.P if your data set represents the entire population, and you

use STDEV.S if your data set represents only a sample of the popula-

tion.

Theories About Standard Deviation

There are many theories about standard deviation. One general rule states

that 95% of a population is located within two standard deviations of the

mean. If you extend your range to within three standard deviations of the

mean, that range should encompass 99.7% of the population.

Figure 14.11
shows the lengths of fish. Column A contains the lengths of all

20 fish in one particular tank at a science museum. Column E contains the

lengths of 20 random fish observed while snorkeling at a coral reef. Both

groups have a mean value of 18.58 inches, as shown in cells C4 and G4.