Six Sigma Basics
What is the Six Sigma Green Belt certification scope?
Attribute charts (p, np, c, u) track discrete data such as pass/fail or defect counts. Variable charts (X-bar, R, S, I-MR) track continuous measurements like length, weight, or temperature.
Critical-to-Quality tree. It translates high-level customer needs (VOC) into specific, measurable, actionable requirements. Steps: customer need → CTQ → measurable requirement → specification.
A t-test compares the means of one or two groups to determine whether differences are statistically significant. Variants include one-sample, two-sample (independent), and paired t-tests.
Green Belt certification covers DMAIC methodology, basic statistics, control charts, and project participation. Green Belts lead small projects and assist Black Belts on larger initiatives.
Six Sigma Basics
What is the difference between short-term and long-term capability studies?
Short-term capability (Cp, Cpk) uses data collected under controlled conditions reflecting only common-cause variation. Long-term capability (Pp, Ppk) uses data spanning a longer period including routine variation.
An R-chart monitors process variability by plotting the range (max-min) of each subgroup over time, used together with the X-bar chart for variables data.
A Gantt chart displays the project schedule as horizontal bars across a timeline, showing task durations, dependencies, milestones, and resource assignments for tracking DMAIC execution.
Poka-yoke is mistake-proofing a process to prevent errors; 5S (Sort, Set, Shine, Standardize, Sustain) is a workplace organization methodology for efficiency.
Six Sigma Basics
What is the role of a Sponsor in a Six Sigma project?
Standardization updates standard operating procedures, work instructions, and training materials to reflect the improved process. It ensures consistent execution, prevents regression, and institutionalizes the gains across the organization.
Periodically evaluating workplace organization against Sort, Set, Shine, Standardize, and Sustain standards to maintain discipline and identify improvement opportunities.
A senior leader who champions the project, removes organizational barriers, secures resources, and ensures alignment with strategic business objectives.
Verification confirms a product or process meets specified requirements (are we building the product right?). Validation confirms it meets the actual user need (are we building the right product?).
Six Sigma Basics
What is the difference between an Xbar-R and Xbar-S chart?
Defects, Overproduction, Waiting, Non-utilized talent, Transportation, Inventory, Motion, and Extra processing (often remembered as DOWNTIME).
Xbar-R uses subgroup ranges and is preferred when subgroup size n ≤ 10. Xbar-S uses subgroup standard deviations and is more accurate when n > 10.
A pilot study tests the proposed solution on a small scale to verify effectiveness, identify unforeseen issues, refine procedures, and build evidence. Successful pilots justify broader rollout and resource commitment.
The number of defects per unit when subgroup sizes vary, normalizing count data across differing opportunity windows.
Six Sigma Basics
What is the Improve phase goal?
To test proposed solutions on a small scale before full implementation, validating effectiveness, identifying issues, and refining the change before rollout.
The Improve phase generates, pilots, and implements solutions that address root causes identified in earlier phases. It focuses on selecting solutions that reduce defects, variation, or cycle time and validating results with data.
Common cause is inherent, random process variation; special cause is an external, assignable factor producing non-random patterns that must be investigated.
Cp measures potential process capability assuming the process is centered on specification. Cpk measures actual capability, accounting for any shift of the mean toward one specification limit.
Six Sigma Basics
What is Standardization in the control phase?
Black Belt certification requires mastery of advanced statistics, DOE, regression, hypothesis testing, and full DMAIC leadership. Candidates must complete a project with documented financial results.
A defect opportunity is any instance where a unit could fail to meet a customer requirement. For a bill, opportunities might include correct name, amount, address, and date—each counted separately for DPMO calculation.
Inventory waste is excess raw materials, work-in-process, or finished goods beyond immediate need. It ties up capital, occupies space, and can mask underlying process problems.
Standardization updates standard operating procedures, work instructions, and training materials to reflect the improved process. It ensures consistent execution, prevents regression, and institutionalizes the gains across the organization.
Six Sigma Basics
What is the control phase's documentation deliverable?
Stratified sampling divides a heterogeneous population into homogeneous subgroups (strata) and randomly samples within each, ensuring representation and reducing sampling error.
A completed project charter that clearly defines the problem statement, scope, goals, timeline, team members, and stakeholders, formally authorizing the project.
The control plan documents the updated process, including control points, responsible owners, reaction plans for out-of-control conditions, training requirements, and metrics for ongoing monitoring and sustainment.
Skills utilization waste (the eighth muda) occurs when organizations underuse employee talents, knowledge, and creativity—failing to listen, train, or engage workers in improvement.
Six Sigma Basics
What is the Define phase's primary deliverable in a DMAIC project?
Lean Six Sigma combines Lean's waste elimination with Six Sigma's variation reduction. It delivers faster cycle times, lower costs, and higher quality by addressing both process efficiency and statistical control simultaneously.
A completed project charter that clearly defines the problem statement, scope, goals, timeline, team members, and stakeholders, formally authorizing the project.
Cycle time is the time to complete one unit from start to finish. Lead time is the total time from customer order to delivery, including wait, queue, and processing times.
Continuous data can take any value within a range (length, temperature). Discrete data has countable values (defects per unit, number of complaints). Continuous data provides more information but requires different statistical tools.
Six Sigma Basics
What is the purpose of a Fishbone diagram in the Analyze phase?
Also called an Ishikawa or cause-and-effect diagram, it helps teams brainstorm and categorize potential root causes of a problem, often grouped into the 6 Ms: Man, Machine, Material, Method, Measurement, and Mother Nature.
Xbar-R uses subgroup ranges and is preferred when subgroup size n ≤ 10. Xbar-S uses subgroup standard deviations and is more accurate when n > 10.
A CTQ tree translates broad VOC needs into measurable requirements through layers: customer needs, CTQs, and measurable specifications with units, tolerances, and operational definitions.
Failure Mode and Effects Analysis. A systematic method to identify potential failure modes in a process or product, score Severity, Occurrence, and Detection to compute a Risk Priority Number, then prioritize actions.
Six Sigma Basics
What is the Voice of the Customer (VOC)?
A run chart plots data over time with a median line to detect trends. A control chart adds statistically derived upper and lower control limits (typically ±3σ) to distinguish common from special cause.
RTY is the probability that a single unit passes through an entire multi-step process defect-free. It is calculated as the product of individual step yields: RTY = Y1 × Y2 × ... × Yn.
The five-number summary (minimum, Q1, median, Q3, maximum) and potential outliers, useful for comparing distribution spread and skewness across groups.
A structured process of capturing customer expectations, preferences, and complaints through interviews, surveys, focus groups, and market research.