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Statistics Resources

This guide contains all of the ASC's statistics resources. If you do not see a topic, suggest it through the suggestion box on the Statistics home page.

One-Tail vs Two-Tail Tests

Two-tailed Test

When testing a hypothesis, you must determine if it is a one-tailed or a two-tailed test. The most common format is a two-tailed test, meaning the critical region is located in both tails of the distribution. This is also referred to as a non-directional hypothesis.

Normal curve showing two-tails shaded in red

This type of test is associated with a "neutral" alternative hypothesis. Here are some examples:

  • There is a difference between the scores.
  • The groups are not equal.
  • There is a relationship between the variables.


One-tailed Test

The alternative option is a one-tailed test. As the name implies, the critical region lies in only one tail of the distribution. This is also called a directional hypothesis. The image below shows a right-tailed test. A left-tailed test would be another type of one-tailed test.

Normal Curve showing one tail shaded in red

This type of test is associated with a more specific alternative claim. Here are some examples:

  • One group is higher than the other.
  • There is a decrease in performance.
  • Group A performs worse than Group B.