Hello Charles. I’m doing comparing water quality before 2010 and after 2010. I’m using t-test from excel to test it. I also used SPSS to run Mann-whitney U-test on the same data. From SPSS, one of the results showed : “Exact Sig. [2*(1-tailed Sig.)]”. From my understanding, I think SPSS determine it as a one-tailed and thus i need to divide the p-value by 2 in order to get the right . But how do i know that the results from t-test in excel is in one tailed or two-tailed? If mann-whitney analysis it as one-tailed, should i take the one-tailed p-value for t-test too (say I run on the same set of data).