The purpose of this chapter is to provide some very brief guidelines for using the power tables in this book. We will illustrate their application within the context of the t-test tables presented at the end of Chapter 4 and a number of common sense principles.

Basically the power tables provided in the chapters which follow may be used to estimate (a) power given an available sample size and hypothesized ES, (b) the sample size required to achieve a given level of power for a hypothesized ES, or (c) the minimum detectable ES for a given level of power and sample size. (For convenience, exact sample size tables are also provided for the most common targeted power levels, ES values, and significance levels.) The following examples illustrate how the power tables may be used for each of these purposes.

Example 1: Calculating power. Suppose that an investigator (a) “knew” (we seldom know anything with any degree of certainty before we conduct an experiment) that 45 subjects per group would be available for his/her two-arm study, (b) estimated that his/her ES would be approximately 0.60 (i.e., the experimental group would differ from the control group by slightly more than one-half of a standard deviation), and (c) wished to know how much power would be available for the experiment.