This page contains references that may interest you if you wish to explore an issue further. None of these are required.

Perspectives on statistical power:

• Cousens, R. and Marshall, C. 1987. Dangers in testing statistical hypotheses. Annals of Applied Biology 111:469-476.
  Review of type I and type II errors and why 'no evidence of a difference' is not the same as no difference.
• Knapp, T.R. 1996. The overemphasis on power analysis. Nursing Research 45:379-381.
  Commentary on how power analysis can be misused. Some is specific to nursing and NIH grants; most applies to any experiment.
• Thomas, L. 1997. Retrospective power analysis. Conservation Biology 11:276-280.
  Good introduction to power analysis. I have focused on using power to design a new experiment (i.e. a prospective analysis). Some folks like to use power to interpret already collected data. This has some problems, which are nicely reviewed by Thomas.