In multiple comparison procedures, family-wise 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 comparison-wise type I error, then the probability aFW of family-wise type I error is usually calculated as follows:
| aFW = 1 – (1 – a)C. |
2
| C = | k(k–1) | . |
For example, for k=4 populations, there are C=6=4(4-1)/2 pairs of populations; and for a = 0.01 we have from the formula – aFW=0.0585 – that is, the probability of family-wise type I error is much higher than the probability of comparison-wise type I errors.
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