We also need to think about statistical power when we fail to reject the null hypothesis. Remember that, when we reject the null, we can have two possible outcomes. We can reject the null because there is no effect in the population (this would be a correct result) or we could be making a Type II error. In this case, we would not have enough sufficient statistical power for the test. How do we know which one is operating? We can never know for certain, but, if we calculate our effect size, we can get a clue. If our effect size is very small, then there is a good chance that there is no effect in the population. If our effect size is moderate to large, then we most likely do not have enough statistical power in our study.