Answer:
Rejecting or Failing to reject the Null Hypothesis (Ho) depends of the P-Value. Generally, the P-value (probability( Observation | Ho ) ) is around .05, thus minimizing the Type 1 error rate. If the P-value < Alpha , you would reject the Ho, and instead believe the Ha (Alternative Hypothesis), and if the P-value > Alpha, you would Fail to reject the Ho because there is not enough evidence to believe the Ha.