Lean Six Sigma Green Belt Certification Lean Six Sigma Green Belt Measure Phase and Statistics 1 — Questions and Answers
Question 1: Which process capability index accounts for BOTH the spread of a process AND how well it is centered relative to the specification limits?
- Cp
- Cpk (Correct answer)
- Pp
- Sigma level
Correct answer: Cpk
Cpk measures both process spread and centering by taking the minimum of the upper and lower capability indices. Cp only measures spread relative to tolerance width but ignores whether the process is centered between the specification limits.
Question 2: In a normal distribution, approximately what percentage of data falls within two standard deviations (±2σ) of the mean?
- 68.27%
- 95.45% (Correct answer)
- 99.73%
- 50.00%
Correct answer: 95.45%
The empirical rule states that approximately 68.27% of data falls within ±1σ, 95.45% within ±2σ, and 99.73% within ±3σ of the mean in a normal distribution. These are foundational values in statistical process control.
Question 3: What is the primary distinction between precision and accuracy in a measurement system?
- Precision measures closeness to the true value; accuracy measures repeatability
- Accuracy measures closeness to the true value; precision measures repeatability and consistency (Correct answer)
- Precision and accuracy are interchangeable terms in MSA
- Accuracy refers to the number of measurements taken; precision refers to instrument calibration
Correct answer: Accuracy measures closeness to the true value; precision measures repeatability and consistency
Accuracy (bias) describes how close measurements are to the actual or reference value, while precision (repeatability/reproducibility) describes how consistently the measurement system produces the same result under the same conditions. A system can be precise but inaccurate (consistently wrong).
Question 4: Which control chart is most appropriate for monitoring the number of defects per unit when sample sizes vary between inspection periods?
- p-chart
- c-chart
- u-chart (Correct answer)
- np-chart
Correct answer: u-chart
The u-chart (defects-per-unit chart) is used for count data (defects) when sample sizes are unequal, because it normalizes defects by dividing by sample size. The c-chart requires a constant sample size, the p-chart tracks proportion defective (not defects), and the np-chart also requires a fixed sample size.
Question 5: What is the primary purpose of establishing an operational definition before collecting data in the Measure phase?
- To identify which statistical tool will be used for analysis
- To ensure every person collecting data measures the same characteristic in exactly the same way (Correct answer)
- To determine the sample size needed for statistical significance
- To establish the upper and lower specification limits for the process
Correct answer: To ensure every person collecting data measures the same characteristic in exactly the same way
An operational definition provides a precise, unambiguous description of what is being measured and how it is measured, ensuring consistency across different operators, times, and locations. Without it, measurement variation introduced by differing interpretations inflates overall process variation.
Question 6: A process produces 3.4 defects per million opportunities (DPMO). At what sigma level is this process operating?
- 3 Sigma
- 4 Sigma
- 5 Sigma
- 6 Sigma (Correct answer)
Correct answer: 6 Sigma
3.4 DPMO is the benchmark performance of a Six Sigma process. This figure accounts for the industry-standard 1.5-sigma long-term shift in the process mean, meaning a process centered at 6σ from the nearest specification limit yields approximately 3.4 DPMO in practice.
Lean Six Sigma Green Belt Certification Lean Six Sigma Green Belt Measure Phase and Statistics 1
Which process capability index accounts for BOTH the spread of a process AND how well it is centered relative to the specification limits?