One-tailed vs. two-tailed tests

• Two-side tests are conventionally used because
  most of the time, we are not sure of the direction
  of the difference


• One-tailed test are used only when we can
  anticipate a priori the direction of a difference


• One-tailed tests are tempting because they are
  more likely to give a significant result


• Given the same z-score, the P-value is halved for
  one tailed test


• It also mean that they run a greater risk of
  rejecting the Null Hypothesis when it is in fact
  correct --- type I error