If your hypothesis is directional you should check the critical values for a one-tailed test. If your hypothesis is non-directional you should check the critical value for a two-tailed test. Take a careful look at some critical values tables and you will notice that it is more difficult to obtain a significant result with a two-tailed test. This is because the values are greater than the one-tailed test values for Spearman’s Rank and Chi Squared and less than the one-tailed value for Wilcoxon signed ranks.