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Exact Test Fisher’s Statistics

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1 Exact Test Fisher’s Statistics

2 Exact Tests Favorable Unfavorable Total Test Control 10 2 4 12 6 12 6
4 12 6 18 A test treatment and a control are compared to determine whether the rates of favorable response are the same. The sample sizes requirements for the chi-square tests are not met by these data

3 if you can consider the margins (12, 6, 12, 6) to be fixed, then you can assume that the data are distributed hypergeometrically and write Pr(nij) = n1+!n2+!n+1!n+2!/n!n11!n12!n21!n22! p-value is the probability of the observed data or more extreme data occurring under the null hypothesis With Fisher’s exact test, determine the p-value for this table by summing the probabilities of the tables that are as likely or less likely, given the fixed margins.

4 The following table includes all possible table configurations and their associated probabilities.
Table Cell (1,1) (1,2) (2,1) (2,2) Probabilities To find the one-sided p-value, you sum the probabilities as small or smaller than those computed for the table observed, in the direction specified by the one-sided alternative. In this case, it would be those tables in which the Test treatment had the more favorable response, or p = =

5 To find the two-sided p-value, you sum all of the probabilities that are as small or smaller than that observed, or p = =


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