Counts per Unit

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.

Although the number of defects observed in an area of a given size is often assumed to have a Poisson distribution, it is understood that the area and count are large enough to support a normal approximation.

Consider the following questions:

•

Is the defect density within prescribed limits?

•

Is the defect density greater than or less than a prescribed limit?

In the Counts per Unit window, options include:

Alpha

is the significance level of your test. The default value is 0.05.

Baseline Count per Unit

is the number of targeted defects per unit. The default value is 0.1.

Difference to detect

is the smallest detectable difference to test against and is specified in defects per unit, or is left blank for computation.

Sample Size

is the sample size, or is left blank for computation.

Power

is the desired power, or is left blank for computation.

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.