# Two Correlations: Power Calculation Results - Quick Tab

Calculating Power

Select the Quick tab of the Two Correlations: 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 vs. N1, and Power vs. N2 power charts.

End N. In the End N box, enter the final value of N used in the Power vs. N, Power vs. N1, and Power vs. N2 power charts.

Start Rho1. In the Start Rho1 box, enter the starting value of rho1 used in the Power vs. Rho1 power chart.

End Rho1. In the End Rho1 box, enter the final value of rho1 used in the Power vs. Rho1 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, to plot on the X-axis in the power charts specified in the Power Charts group box.

Computational Algorithm. The Power Analysis module allows you to choose from two algorithms:

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 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 r, and that the variance of the Fisher transform of r is 1/(N - 3).

Power Charts. Click the buttons in the Power Charts box to produce charts of power as a function of variables known to affect it. Note that parameters given in the X-Axis Graphing Parameters box 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 assuming equal sample sizes in each of the two independent samples on which the Z-statistic is based.

Power vs. N1. Click the Power vs. N1 button to produce a plot of power versus group 1 sample size assuming the sample size in the second group is held constant at the current fixed parameter value.

Power vs. N2. Click the Power vs. N2 button to produce a plot of power versus group 2 sample size assuming the sample size in the first group is held constant at the current fixed parameter value.

Power vs. Rho1. Click the Power vs. Rho1 button to produce a plot of power versus population correlation in the first group.

Power vs. Alpha. Click the Power vs. Alpha button to produce a plot of power vs. type I error rate (a).