Estimating Power (1 of 4)
The primary purpose of power analysis is to guide in the choice of
sample size. First, the experimenter
specifies the power that he or she wishes to achieve. Then, the
sample size needed for that level of power can be estimated. The
calculations for power depend on the size of the effect in the
population. Therefore, the first and most
difficult step in choosing a sample size is to estimate the size of
the effect. If there are published experiments similar to the one to
be conducted, then the effects obtained in these published studies
can be used as a guide to the size of the effect. There is a need for
caution, however, since there is a tendency for published studies to
contain
overestimates of effect sizes.
Often previous studies are not sufficiently similar to a new study to
provide a valid basis for estimating the effect size. In this case,
it is possible to specify the minimum effect size that is considered
important. For example, an experimenter interested in the
effectiveness of a course that prepares students for the quantitative
portion of the SAT
might consider a difference of 30 points between the treatment
group and the control group to be the minimum effect size worth
detecting.