The one versus the two-tailed test

Obviously, what we're shooting for is to be able to reject a null hypothesis so we can say our research hypothesis is correct, or might be correct, at least is potentially correct.

So the bigger the rejection area (for we're testing to reject the null hypothesis) the better. That's why social workers tend to set their alphas at .05, not .01.

Another way to do this is to use a one-tailed test. In the two-tailed, non-directional tests, the rejection area, that five percent, is divided into .025 and .025. We're testing whether or not the intervention helped, for example, or perhaps if it hurt.

There isn't always a purpose to see it something hurt, so we can use a directional, as opposed to a non-directional test. This way we set the alpha at .05 and the rejection area seems larger because it's all at one side of the curve, 1.65 SD from the mean, as opposed to 1.96 SD in a 2-tailed test.

That increased the power, or the likelihood the test might reject the null, by increasing the size of the rejection region.