Prospective versus retrospective assessment of power

You can examine power and sample size issues before you design and run an experiment (prospective) or after you perform an experiment (retrospective).

·    A prospective study is used before collecting data to consider design sensitivity. You want to be sure that you have enough power to detect differences (effects) that you have determined to be important. For example, you can increase the design sensitivity by increasing the sample size or by taking measures to decrease the error variance (standard deviation).

·    A retrospective study is used after collecting data to help understand the power of the tests that you have performed. For example, suppose you conduct an experiment and the data analysis does not reveal any statistically significant results. You can then calculate power based on the minimum difference (effect) you wish to detect. If the power to detect this difference is low, you may want to modify your experimental design to increase the power and continue to evaluate the same problem. However, if the power is high, you may want to conclude that there is no meaningful difference (effect) and discontinue experimentation.