1. Power and Sample Size David M. Thompson, Ph.D., P.T.
Associate Professor Dept. of Biostatistics and Epidemiology College of Public Health, OUHSC

2. Power and sample size calculations
EFFECT
SIZE
Specify or estimate any two to calculate the third.

3. Power
The probability of rejecting the null hypothesis when a specified alternative is true.
Both null and alternative hypotheses must be specified; the alternate hypothesis specifies a clinically meaningful effect size.

4. Sample Size To calculate sample size, first must specify: 
Power = (1-)
Effect size
Difference in means
Difference in proportions

5. Effect size
Meaningful and detectable difference in group means, proportions, etc. …
…including estimate of within group variability

6. In study planning:
Power is customarily prespecified at .8 or .9
A minimal clinically important difference focuses the decision on the effect size, from which we calculate sample size.
If practical conditions constrain the sample size, we specify that, then calculate the effect size we have prespecified power to detect with that sample.

7. Do it yourself resources
Web primer on “hypothesis testing and statistical power”
moon.ouhsc.edu/dthompso/CDM/power/hypoth.htm
Java applets that explore influences of sample and effect sizes on power
Power and sample size programs (UCSF)