If in doubt, choose a two-tail P value. Why?
•The relationship between P values and confidence intervals is easier to understand with two-tail P values.
•Some tests compare three or more groups, which makes the concept of tails inappropriate (more precisely, the P values have many tails). A two-tail P value is more consistent with the P values reported by these tests.
•Choosing a one-tail P value can pose a dilemma. What would you do if you chose to use a one-tail P value, observed a large difference between means, but the “wrong” group had the larger mean? In other words, the observed difference was in the opposite direction to your experimental hypothesis. To be rigorous, you must conclude that the difference is due to chance, even if the difference is huge. While tempting, it is not fair to switch to a two-tail P value or to reverse the direction of the experimental hypothesis. You avoid this situation by always using two-tail P value.