p-value for one-tailed test I have some confusion in interpreting p-value from SPSS.
If my hypothesis is one-tailed, after getting the p-value from SPSS, since the p-value from SPSS is always two-tailed,  
ie. p = 0.06 (from SPSS), alpha = 0.05
In this case, does it means that my p-value will become 0.03 and significant? or does it means that I need to use p < 0.025 as my cut-off point to consider the result as significant?
Thanks a lot.
 A: Generally speaking, if the p value is calculated from a two tailed test you will need to divide it by two if you want to use it for a one tailed test as the area under the probability curve is divided by two as well.

In the picture you can see the area "used" for a right tailed test. A p value in a two sided test is the marginal significance of both sides added together. So in a two tailed test with a p-value of 0.02 the marginal significance on the right and left side would both be 1%. 
With that being said, I would be careful with using the p-value of a two tailed test to reject the null of a one tailed test. If you simply say 0.03 < 0.05 thus you can reject the H0 then you could reject the H0 for both, a right sided and a left sided test. In terms of interpretation this does not make sense to me as you would say the estimate is significantly larger than the H0 while at the same time being significantly smaller than the H0.
Instead I would recommend using the t-values for your test decision. More specifically, 
if the t-value is larger than then critical t-value (the t value obtained at a 95% area under the probability curve (or the t-value of two sided test with alpha = 0.1) and the degrees of freedom of your test)  and you do a right sided test then you can reject the H0. Note that you do not take the absolute t-value if you do a one tailed test.
For a left sided test you can reject the H0 if the t-value is smaller than the critical t-value (at 5%) or, if you feel lazy, just -(the critical t-value at the 95% level from the right sided test) since the t and z distributions are symmetric. 
I don't use SPSS so I don't know whether it actually gives you a two-tailed test p-value if you define the H1 and H0 as the proper side one tailed test. 
Hope this helps.
