Re: 1-tailed test and alpha

Posted by Richard Ristow on Oct 23, 2006; 7:45pm
URL: http://spssx-discussion.165.s1.nabble.com/1-tailed-test-and-alpha-tp1071625p1071627.html

At 09:50 AM 10/23/2006, Helga Walz wrote:

>SPSS provides probabilities for 2-tailed tests only.
>
>With a 1-tailed test and alpha 0.05, I understand that I need to
>divide the Sig. (2-tailed).
>
>Thus, if SPSS list the p-value as 0.881 [I assume you mean .0881],
>that means for the 1-tailed test the p-value is 0.04405.

To give a side piece of advice: consider VERY carefully before doing a
one-tailed test. 95% of the time, stay with the two-tailed test. The
one-tailed test has more statistical power, but very marginally so. And
the one-tailed test is valid only if a result in the unexpected
direction is truly of no interest - a rare case.