Randomization Tests (3 of 6)

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To compute the probability value for a one-tailed test of the difference between groups, divide this value of three by the W of ways of dividing the data into two groups of four. The probability value is therefore: p = 3/70 = 0.0429.

For a two-tailed test, the three cases in which Group 2 had a mean that was greater than Group 1 by six or more would be considered. This would make the two-tailed probability: p = 6/70 = 0.0857.

In summary, a randomization test proceeds from the data actually collected. It compares a computed statistic (the difference between means in this example) with the value of that statistic for other arrangements of the data. The probability value is simply the proportion of arrangements leading to a value of the statistic as large or larger than the value obtained from the actual data.