You can use the Counts per Unit option from the Sample Size and Power window (Sample Size and Power Choices) to calculate the sample size needed when you measure more than one defect per unit. A unit can be an area and the counts can be fractions or large numbers.
In the Counts per Unit window, enter Alpha and the Baseline Count per Unit. Then enter two of the remaining fields to see the calculation of the third. The test is for a one-sided (one-tailed) change. Enter the Difference to Detect in terms of the Baseline Count per Unit (defects per unit). The computed sample size is expressed as the number of units, rounded to the next whole number.
Leave Alpha as 0.05 (the chance of failing the test if the new process is as good as the target).
Enter 4 as the Baseline Counts per Unit, indicating the target of 4 defects per wafer.
Enter 1 as the Difference to detect.
Click Continue to see the results in Window For Counts Per Unit Example, showing a computed sample size of 38 (rounded to the next whole number).
Window For Counts Per Unit Example