Statistical power To understand power, look at the distribution of the experimental group. See how much of the area of that distribution is beyond the vertical line corresponding to the 0.05 alpha level. In the first figure, the entire distribution is to the right of the vertical line, therefore power is maximum. In the second figure, only part of the distribution is to the right of the vertical line. Obviously, power is reduced when the difference between the two groups is reduced.
Power shows the capability of an experiment to detect a difference of a certain magnitude.
The larger the difference between the experimental group and the control group, the less overlap between the two groups, and the more power for detecting the difference. The greater the overlap, the lower the power. In effect, large differences will be easy to detect and small differences will be difficult to detect. An understanding of these performance characteristics is important for both planning an experiment and for interpreting the results of an experiment. You are in control of some important factors when you design an experiment.