The power of a study is the ability of the study to detect a significant change when one exists. It is calculated as 1 - b, where b is the Type II error. In this case, 1 - b = 0.80, or 80%. Therefore, there is an 80% surety that this study has detected a change in mortality with this new treatment when one exists. Or, in other words, 20% of the time it will have missed a significant difference when one exists.