Calculating Required Sample Size

Select the Quick
tab of the Comparing 2 Proportions: Sample Size Calculation
Results dialog box to access options to interactively view the results
of sample size calculations and generate plots of required sample size
as a function of quantities that affect sample size 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 Sample Size Charts.

Start Power. In the Start Power box, enter the starting value of the power goal used in the N vs. Power sample size chart.

End Power. In the End Power box, enter the final value of the power goal used in the N vs. Power sample size chart.

Start Pi1. In the Start Pi1 box, enter the starting value of Pi1 used in the N vs. Pi1 sample size chart.

End Pi1. In the End Pi1 box, enter the final value of Pi1 used in the N vs. Pi1 sample size chart.

Start Alpha. In the Start Alpha box, enter the starting value of Alpha to use in the N vs. Alpha sample size chart.

End Alpha. In the End Alpha box, enter the final value of Alpha to use in the N vs. Alpha sample size 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 charts specified in the Sample Size Charts group box.

N to Vary. Use the options under N to Vary to control which sample size is varied when an optimal sample size is determined. Depending on the options selected, you can vary both N1 and N2 simultaneously, vary either one (while the other is held fixed), or vary both in a constant ratio.

Equal N. Select the Equal N option button to vary both N1 and N2 simultaneously.

Vary N1. Select the Vary N1 option button to vary N1 while holding N2 constant at its current fixed parameter value.

Vary N2. Select the Vary N2 option button to vary N2 while holding N1 constant at its current fixed parameter value.

Fixed Ratio. Select the Fixed Ratio option button to vary N1 and N2 in fixed ratio. If this option button is selected, the program searches for N in the following way: It selects an N2, then multiplies it by the N1/N2 ratio (converting to an integer) to obtain N1. Power is then calculated. Values of N2 are searched in this fashion until the smallest value achieving the desired power is obtained.

(N1/N2). Enter the desired ratio
N1/N2 in the (N1/N2) field.

Sample Size Charts. Use the buttons under Sample Size Charts to produce charts of required N as a function of power goal, population proportion, or type I error rate. Note that parameters given in the X-Axis Graphing Parameters group box control the range of values plotted on the X-axis and the number of steps plotted.

N vs. Pi1. Click the N vs. Pi1 button to produce a plot of sample size vs. Pi1.

N vs. Power. Click the N vs. Power button to produce a plot of sample size (N) versus power goal.

N vs. Alpha. Click the N vs. Alpha button to produce a plot of sample size (N) vs. type I error rate.