Hi WeeG

It is nice to meet a new contributor who is actually pretty older!

I guess if we predetermine our hypothesis in a one-way direction (for example whether A is necessarily greater than B or not [instead of whether A and B are different in any direction or not]), the alpha should remain at 0.05. However, I suppose some people might cheat this and run a one-sided test, AFTER obtaining the results. Their reason can be to boost the power of their test and obtain a significant P value, as a one-sided test will output a P value half the value of the P value from the same comparison but using a two-sided test.

So I guess, some authorities might want to neutralize this effort by already dividing the routine alpha (0.05) by 2, to offset for the abovementioned way of reducing the P value. In this case, using a one-sided test will not help any authors to suddenly obtain a significant P value, when their real two-sided test was not significant.