**What is the difference between a one-tailed and a two-tailed test of significance? Under what circumstances would each be used?**

According to
Aron, Aron, & Coups (2009), a one –
tailed test is a hypothesis – testing procedure where the region for rejection
of the null hypothesis is on one side of the distribution. Also according to Aron, Aron, & Coups (2009), a two – tailed
test is a hypothesis – testing procedure where the region for rejection of the
null hypothesis is on both sides of the distribution. One – tailed tests are
for directional hypothesis (predicting a particular difference); two – tailed
tests are for non directional hypothesis (predicting no particular difference).

Suppose
researchers are testing a new drug, Drug Z, and its effects. If the hypothesis
was, “Drug Z is just as effective as similar drugs on the market.”, a one –
tailed test would be appropriate. In testing this hypothesis, the researchers
are concerned with whether or not Drug Z is at least equal to the other drugs.
They are not concerned with the significance levels above or below the other
drugs. However, if researchers were testing how Drug Z compared with other
drugs a two – tailed test would be appropriate. In this study, the researchers
would want to know how significantly above or below the effectiveness of other
drugs that Drug Z was.

Aron, A., Aron, E. N., & Coups, E. J. (2009).

*Statistics for psychology (5th ed).*Upper Saddle River, NJ: Pearson/Prentice Hall.**Plagiarism:**

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