Statistics Dictionary

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Error Rate Familywise

Error rate refers to the probability of making a Type I error - rejecting the null hypothesis when it is true. When an experiment tests multiple comparisons, researchers need to be aware of two types of error rates:

  • Error rate per comparison. When a null hypothesis is represented by a single comparison, error rate per comparison is the probability of making a Type I error when testing the comparison.
  • Error rate familywise. A family of comparisons refers to all of the comparisons defined for a single treatment in an experiment. Error rate familywise is the probability of making at least one Type I error when testing the family of comparisons.

Both error rates are controlled by the experimenter. Error rate per comparison is determined by the significance level (α) specified for individual hypothesis tests. And error rate familywise is determined by the significance level and the number of comparisons in the family.

See also:  Multiple Comparisons