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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.
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