Factors Affecting Power
Prerequisites
Introduction
to Power,
Example Calculations, Significance
Testing, Type I and Type
II Errors, One and TwoTailed Tests
Learning Objectives
 State five factors affecting power
 State what the effect of each of the factors is
Several factors affect the power
of a statistical test. Some of the factors are under the control
of the experimenter whereas others are not. The following example
will be used to illustrate the various factors.
Suppose
a math achievement test were known to be normally
distributed with a mean of 75 and standard
deviation of σ.
A researcher is interested in whether a new method of teaching
results in a higher mean. Assume that although the experimenter
does not know it, the population mean μ is
larger than 75. The researcher plans to sample N subjects and
do a onetailed test of the whether the sample mean is significantly
higher than 75. What is the probability that the researcher will
correctly reject the false null hypothesis that
the population mean
is 75 at the 0.05 level?
Sample Size
Figure 1 shows that the
larger the sample size, the higher the power. Since sample size
is typically under an experimenter's control, increasing sample
size is one way to increase power. However, it is sometimes
difficult and/or expensive to use a large sample size.
Standard Deviation
Figure 1 also shows that power is higher when
the standard deviation is small than when it is large. Experimenters
can sometimes control the standard deviation by sampling from
a homogeneous population of subjects or by reducing random measurement
error.
Difference between Hypothesized and True Mean
Naturally, the larger the effect size, the more
likely it is that an experiment would find a significant effect.
Figure 2 shows the effect of increasing the difference between
the mean specified by the null hypothesis (75) and the population
mean μ for standard deviations of 10 and 15.
Significance Level
There is a tradeoff between the significance level and power:
the more stringent (lower) the significance level, the lower the
power. Figure 3 shows that power is lower for the 0.01 level than
it is for the 0.05 level. Naturally, the stronger the evidence
needed to reject the null hypothesis, the lower the chance that
the null hypothesis will be rejected.
One versus TwoTailed Tests
Power is higher with a onetailed test
than with a twotailed test as long as the hypothesized direction
is correct. A onetailed test at the 0.05 level has the same power
as a twotailed test at the 0.10 level. A onetailed test, in
effect, raises the significance level.
