This feature requires IBM® SPSS® Statistics Base
Power analysis plays a pivotal role in a study plan, design, and conduction.
The calculation of power is usually before any sample data have been collected, except possibly from
a small pilot study. The precise estimation of the power may tell investigators how likely it is
that a statistically significant difference will be detected based on a finite sample size under a
true alternative hypothesis. If the power is too low, there is little chance of detecting a
significant difference, and non-significant results are likely even if real differences truly
In paired-samples analysis, the observed data contain two paired and correlated samples, and each
case has two measurements. It is assumed that the data in each sample independently and identically
follow a normal distribution with a fixed mean and variance, and draws statistical inference about
the difference of the two means.
- From the menus choose:
- Select a test assumption Estimate setting
(Sample size or Power).
- When Power is selected as the test assumption
Estimate method, enter the appropriate Sample size
value. The value must be a positive integer greater than, or equal to, 2.
- Optionally, select an option from the Specify
- Hypothesized Values
- The default setting provides the Mean and
Standard Deviation settings.
- Population mean difference
- When a single population mean is required, enter a Population
mean difference value. When single value is specified, it denotes the population mean
- Population mean for group 1 and group 2
- When multiple population means are required for the specified group pairs,
enter values for Population mean for group 1 and Population mean
for group 2. When multiple values are specified, they denote the population mean
difference μ1 and μ2.
- Population standard deviation for mean difference
- When a single population mean is specified, enter the population standard
deviation for mean difference value. When a single value is specified, it denotes the population
standard deviation of the group difference σd. The value must be a single numeric
greater than 0.
- Population standard deviation for group 1 and group 2
- When multiple population means are specified, enter the population standard
deviation for group 1 and group 2 values. When multiple values are specified, they denote the
population standard deviation of the group difference σ1 and σ2.
The values must be a single numerics greater than 0.
- Pearson product-moment correlation coefficient
- Optionally, enter a value that specifies the Pearson product-moment
correlation coefficient ρ. The value must be a single numeric value between -1 and 1. The
value cannot be 0.
- Effect Size
- Estimates the effect size as an input to the estimation of the power or
sample size. The defined effect size Value is passed to the intermediate step
in the procedure and calculates the desired power or sample size.
- Select whether the test is one or two-sided.
- Nondirectional (two-sided) analysis
- When selected, a two-sided test is used. This is the default setting.
- Directional (one-sided) analysis
- When selected, power is computed for a one-sided test.
- Optionally, specify the significance level of the Type I error rate for the
test in the Significance level field. The value must be a single double value
between 0 and 1. The default value is 0.05.
- Optionally, click Plot to specify Power Analysis of Paired-Samples T Test: Plot settings (chart output, two-dimensional plot settings,
three-dimensional plot settings, and tooltips).
- Optionally, click Precision to estimate the sample
size based on confidence intervals by specifying the values of the confidence interval half-widths.
For more information, see Power Analysis: Precision.