The F Test is used to compare the variances of two samples to test the hypothesis that the samples are drawn from populations with different variances:
H_{0} the samples are drawn from populations with equal variances
H_{1} the samples are drawn from populations with different variances
The F test is based on the ratio of the variances of the two samples:

1

2

3

4

5

Variance 
Sample A

8.7

9.8

12.2

12.9

9.5

3.327

Sample B

23.8

21.5

18.3

24.3

22.9

5.788

This gives an F statistic:
(Most tables give critical values greater than 1, so divide the larger by the smaller)
This can be compared to the critical F value from tables, or the pvalue can be obtained using the Excel function:
=FDIST(F_{o}, DOF_{n}, DOF_{d}) = 0.302
DOF_{n} and DOF_{d} are the Degrees of Freedom of the numerator and denominator (both are 4 in this example)
The F Test is based on the F Distribution.
