Six Sigma Green Belt Certification Practice Exam

Which formula would you use to calculate the value of sigma for use in calculating Ppk?

1. (USL - X-bar) / 3

2. (USL - LSL) / 6

3. R-bar / d2

4. Square root (Sum (X - X-bar)^2 / (n - 1))

The correct answer is: Square root (Sum (X - X-bar)^2 / (n - 1))

To determine the value of sigma for use in calculating Ppk, the correct approach involves finding the standard deviation of the process or sample data, which is crucial for assessing process capability. The formula involving the square root of the sum of the squared deviations from the mean, divided by the sample size minus one, accurately provides that standard deviation. This formula effectively captures the variability within the data, giving a precise measurement of how spread out the data points are around the mean. Understanding this concept is vital in the context of Ppk, which is used to evaluate how well a process meets specification limits relative to its natural variation. By calculating sigma accurately, you ensure that the Ppk value reflects the true capability of the process, allowing for better decision-making when it comes to process improvement. The other options focus on aspects unrelated to the actual calculation of sigma needed for Ppk. For instance, the option analyzing the difference between the upper specification limit and the mean does not result in the standard deviation required for all Ppk calculations. Similarly, the other formulas do not provide a measure of variability, which is essential for determining process capability in Six Sigma methodology.