t-Test for Zero Correlation: Power Calculation Results - Quick Tab
Calculating Power
Select the Quick tab of the t-Test for Zero Correlation: Power Calculation Results dialog box to access options to interactively view the results of power calculations and generate plots of power as a function of quantities that affect power in practice.
- X-Axis Graphing Parameters
- The entries in the boxes under X-Axis Graphing Parameters establish the start value, end value, and number of steps on the X-axis for the graphs specified under Power Charts.
- Start N
- In the Start N box, enter the starting value of N used in the Power vs. N power chart.
- End N
- In the End N box, enter the final value of N used in the Power vs. N power chart.
- Start Rho
- In the Start Rho box, enter the starting value of rho used in the Power vs. Rho power chart.
- End Rho
- In the End Rho box, enter the final value of rho used in the Power vs. Rho power chart.
- Start Alpha
- In the Start Alpha box, enter the starting value of alpha used in the Power vs. Alpha power chart.
- End Alpha
- In the End Alpha box, enter the final value of alpha used in the Power vs. Alpha power chart.
- No. of Steps.
- In the No. of Steps box, enter the number of steps, or intervals, plotted on the X-axis in the power charts specified in the Power Charts box.
- Computational Algorithm
- Most programs that include methods for power analysis of tests on correlations do not provide exact probability calculations. The Power Analysis module allows you to choose from three algorithms:
- Exact
- Select the Exact option button to obtain exact calculations. These calculations take substantially longer to compute than the approximations given below, but the calculation is not prohibitively time-consuming for the sample sizes typically employed in research studies.
- Fisher Z Refined
- Select the Fisher Z Refined option button to employ an approximation based on the Fisher transformation, but using refined series approximation formulae for the mean and variance (Fouladi, 1991).
- Fisher Z Crude
- Select the Fisher Z Crude option button to use the traditional (somewhat crude) approximation, based on the Fisher transformation, assuming that the mean of the Fisher transform of r is the Fisher transform of ρ, and that the variance of the Fisher transform of r is 1/(N - 3).
- Power Charts
- Use the buttons under Power Charts to produce charts of power as a function of variables known to affect it. Note that the parameters specified under X-Axis Graphing Parameters control the range of values plotted on the X-axis and the number of steps plotted.
- Power vs. N.
- Click the Power vs. N button to produce a plot of power versus sample size N.
- Power vs. Rho.
- Click the Power vs. Rho button to produce a plot of power versus population correlation (ρ).
- Power vs. Alpha.
- Click the Power vs. Alpha button to produce a plot of power vs. type I error rate (α).
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