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Should I use One-tailed or Two-tailed Hypothesis Tests?

By Steve Creech | February 26, 2011

One-sided alternative hypotheses are rarely used and I usually discourage their use. The point is, why limit yourself to a one-sided alternative hypothesis? If the results should happen to be statistically significant, but in the opposite direction you expected, you will not be able to reject the null hypothesis.

It is my understanding there are mainly two circumstances under which you would use a one-tailed test: 1) when a statistically significant result in one of the directions would be of no interest (e.g. a pharmaceutical company developing a new drug and they only care if it is better than the standard drug), or; 2) when it is physically impossible for the relationship to go in one of the directions.

I have worked with many doctoral students that have had a committee member that felt very strongly that one-tailed hypothesis tests should be used. I think the advice is a misguided attempt to get the doctoral student to state their “expected finding”. In other words, sometimes a committee member will say that you should have a “research hypothesis” that specifies which direction the correlation will go.

I think in this context, “research hypothesis” is synonymous with “expected findings”, but not synonymous with “alternative hypothesis”. In other words, what I think you should do is, use two-tailed alternative hypotheses but then add a section called “expected findings”. That way, you can have the full benefit of two-tailed tests and still satisfy the committee member’s request for you to state a research hypothesis that specifies a particular direction.

Topics: Dissertation Advice |

One Response to “Should I use One-tailed or Two-tailed Hypothesis Tests?”

  1. Bijaya Gopal Shrestha Says:
    October 11th, 2011 at 9:41 pm

    Dear Creech,

    I’m very much impressed with the last para:

    “research hypothesis” is synonymous with “expected findings”, but not synonymous with “alternative hypothesis”. In other words, …, use two-tailed alternative hypotheses but then add a section called “expected findings”.

    Regards
    Bijaya G. Shrestha

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