I error, the determination of a clinically important effect, the potential sample size, as well as the importance of identifying an effect, should one exist. Some studies involve putting patients at risk. At one extreme, the risk might involve a loss of time spent completing a questionnaire. At the other extreme, the risk might involve the use of an ineffective treatment for a potentially fatal disease. These issues are clearly beyond the scope of this discussion, but one point should be made here.
Ethical issues play a role in power analysis.
If a study to test a new drug will have adequate power with a sample of 100 patients, then it would be inappropriate to use a sample of 200 patients since the second 100 are being put at risk unnecessarily. At the same time, if the study requires 200 patients to yield adequate power, it would be inappropriate to use only 100. These 100 patients may consent to take part in the study on the assumption that the study will yield useful results. If the study is under-powered, then the 100 patients have been put at risk for no reason. Of course, the actual decision making process is complex.