One-Tailed vs. Two-Tailed Design

If you think you know that the difference between your control and experimental conditions will be in a particular direction, you can use a "one-tailed" design. If you just hope there will be *some* difference, but don't want to commit to what direction the difference will be in, you must use a "two-tailed" design.

  • Do subliminal cues improve memory performance? One-tailed design.
  • Do subliminal cues affect memory performance? Two-tailed design.

One-tailed tests make it easier to reject the null hypothesis, as long as you are correct about which direction the difference lies in.

It is intellectually dishonest to do a one-tailed test if you didn't a priori expect the difference to be in the direction that was then observed.

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