Familywise Type I Error:
In multiple comparison procedures, familywise type I error is the probability that, even if all samples come from the same population, you will wrongly conclude that at least one pair of populations differ.
If a is the probability of comparisonwise type I error, then the probability a_{FW} of familywise type I error is usually calculated as follows:

where C is the total number of pairwise comparisons for k populations:

For example, for k=4 populations, there are C=6=4(41)/2 pairs of populations; and for a = 0.01 we have from the formula  a_{FW}=0.0585  that is, the probability of familywise type I error is much higher than the probability of comparisonwise type I errors.
See also: Bonferroni adjustment.
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