# one-tailed test

### 5 Ways to Increase Power in a Study

June 12th, 2009 by

To increase power:

1. Increase alpha
2. Conduct a one-tailed test
3. Increase the effect size
4. Decrease random error
5. Increase sample size

Sound so simple, right?  The reality is that although these 5 ways all work (more…)

### One-tailed and Two-tailed Tests

November 19th, 2008 by

I was recently asked about when to use one and two tailed tests.

The long answer is:  Use one tailed tests when you have a specific hypothesis about the direction of your relationship.  Some examples include you hypothesize that one group mean is larger than the other; you hypothesize that the correlation is positive; you hypothesize that the proportion is below .5.

The short answer is: Never use one tailed tests.

Why?

1. Only a few statistical tests even can have one tail: z tests and t tests.  So you’re severely limited.  F tests, Chi-square tests, etc. can’t accommodate one-tailed tests because their distributions are not symmetric.  Most statistical methods, such as regression and ANOVA, are based on these tests, so you will rarely have the chance to implement them.

2. Probably because they are rare, reviewers balk at one-tailed tests.  They tend to assume that you are trying to artificially boost the power of your test.  Theoretically, however, there is nothing wrong with them when the hypothesis and the statistical test are right for them.