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IASSC Certified Lean Six Sigma Green Belt (ICGB): free practice test

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Practice test questions with answers and explanations

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01A process has its mean centred between the specification limits and the distance from the mean to the nearest limit equals 6 standard deviations. A colleague adds a 1.5σ long-term shift. How many standard deviations separate the shifted mean from the nearest limit?

Practice question Unit: Define Phase

  1. A4.5 ✓ Correct answer

    With a 1.5σ shift toward the nearest limit, the distance drops from 6σ to 6 − 1.5 = 4.5σ, which gives about 3.4 DPMO.

  2. B6.0

    Six ignores the long-term shift that the colleague added.

  3. C7.5

    Adding the shift moves the mean toward the limit, so the distance shrinks rather than grows.

  4. D9.0

    Multiplying 6 by the 1.5 factor is not how the shift is applied; the shift is subtracted.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 1.1, item 1.1.1, p.7; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 1.1.1

02Customers of an online store say they want 'reliable delivery'. Which CTQ is best specified for a project metric?

Practice question Unit: Define Phase

  1. ACustomers feel that the company keeps its promises about deliveries

    A feeling is not an operational measurement.

  2. BAverage number of days from order to delivery across all orders

    Average time is measurable, but it has no specification tied to the promise, so late orders can hide behind a good average.

  3. CNumber of delivery complaints received by customer service each month

    Complaints are a late, partial signal: many late orders are never reported, so they do not measure reliability directly.

  4. DPercentage of orders delivered within the promised two-day window ✓ Correct answer

    It is measurable, linked to the need and has a clear specification.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 1.2, item 1.2.2, p.7; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 1.2.2

03A call centre fishbone for 'long hold times' lists 'staff are lazy' under People. As facilitator, what is the best way to handle this entry?

Practice question Unit: Measure Phase

  1. AKeep it as written so no contributor feels censored

    Respecting contributors matters, but the entry is untestable and blames people.

  2. BKeep it, but collect each agent's hold times to test it

    Testing is right, but the entry stays a personal judgement; restate it as a process cause such as staffing at peak times.

  3. CMake it the main root cause, since many agents agreed

    Agreement is not verification by data.

  4. DReword it as a testable cause, e.g. peak understaffing ✓ Correct answer

    Causes should be specific, factual and testable rather than blame.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 2.1, item 2.1.1, p.7; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 2.1.1

04Repair times (hours): 4, 7, 7, 9, 10, 12, 30. A manager wants one figure for a 'typical' repair. Which measure is most suitable, and what is its value?

Practice question Unit: Measure Phase

  1. AMean, 11.3

    The mean is distorted by the outlier of 30.

  2. BMode, 7

    The mode is just the most repeated value and may not be typical.

  3. CMedian, 9 ✓ Correct answer

    The value 30 is an outlier that pulls the mean up; the median resists outliers.

  4. DRange, 26

    The range measures spread, not centre.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 2.2, item 2.2.2, p.7; IASSC Examination Reference Document V1.4.2a, Descriptive Statistics, p.4

05Two gauges measure a 25.00 mm reference block 10 times each. Gauge A: mean 25.00, readings range 24.90–25.10. Gauge B: mean 25.04, readings range 25.03–25.05. Which statement is correct?

Practice question Unit: Measure Phase

  1. AA is better, since its mean is on target

    A's mean is on target, but its spread is much wider.

  2. BB is better, since its spread is narrower

    B's spread is narrow, but it is biased by 0.04.

  3. CBoth gauges are equivalent

    They differ in both bias and precision.

  4. DA: accurate, imprecise; B: precise, biased ✓ Correct answer

    A's mean is on target with wide scatter; B is tight but offset by 0.04 mm.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 2.3, item 2.3.1, p.7; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 2.3.1

06A multi-vari study of shaft diameter shows small differences within each shaft and between consecutive shafts, but large jumps between morning and afternoon samples. Where should the team look for the root cause?

Practice question Unit: Analyze Phase

  1. AOut-of-round grinding within an individual shaft

    Within-piece variation was small.

  2. BPiece-to-piece machine vibration

    Consecutive-piece variation was small.

  3. CTime-related factors such as temperature ✓ Correct answer

    The dominant family is temporal, so time-related Xs are the likely causes.

  4. DPoor gauge resolution affecting every reading

    A coarse gauge would blur all readings alike; it would not create a jump between morning and afternoon.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 3.1, item 3.1.1, p.8; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 3.1.1

07A sample of n = 16 has X̄ = 52.0, and the population σ is known to be 6. What is the 95% confidence interval for the mean (Z = 1.96)?

Practice question Unit: Analyze Phase

  1. A40.24 to 63.76

    This uses σ instead of the standard error.

  2. B49.06 to 54.94 ✓ Correct answer

    52.0 ± 1.96 × 6/√16 = 52.0 ± 2.94.

  3. C49.53 to 54.47

    This uses Z = 1.645, the 90% value.

  4. D50.50 to 53.50

    This adds only one standard error (1.5).

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 3.2, item 3.2.1, p.8; IASSC Examination Reference Document V1.4.2a, Hypothesis Testing, p.7; IASSC Examination Reference Document V1.4.2a, Z Table, p.16–17

08For data on machine speed X and output Y: Σ(X − X̄)(Y − Ȳ) = 24, Σ(X − X̄)² = 40 and Σ(Y − Ȳ)² = 15.2. What is r?

Practice question Unit: Improve Phase

  1. A0.039

    This divides by the product without the square root.

  2. B0.600

    This is the slope b = 24/40, not r.

  3. C0.947

    This is R² = r².

  4. D0.973 ✓ Correct answer

    r = 24 / √(40 × 15.2) = 24 / 24.66 = 0.973.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 4.1, item 4.1.1, p.8; IASSC Examination Reference Document V1.4.2a, Regression, p.11

09A plant's 5S scores are always above 95%, but audits are announced a week in advance and the area is often cluttered on other days. What should change?

Practice question Unit: Control Phase

  1. AAudit more often, still with advance notice

    More announced audits still score a prepared area, not the normal daily state.

  2. BKeep the current audits, as the scores show success

    The high scores mislead, because announced audits do not reflect daily conditions.

  3. CSwitch to unannounced audits at random times ✓ Correct answer

    Unannounced checks measure the normal state, not a prepared one.

  4. DRaise the target to 99%

    The method, not the target, is the problem.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 5.1, item 5.1.1, p.9; IASSC Lean Six Sigma Green Belt Body of Knowledge V3.2, item 5.1.1

10An individuals chart has X̄ = 24.9 and MR̄ = 1.5. A new reading of 29.0 arrives. Using limits X̄ ± 3 × (MR̄/1.128), what should happen?

Practice question Unit: Control Phase

  1. AUCL = 27.56; investigate

    This uses 2σ instead of 3σ limits; the decision is right but the limit is wrong.

  2. BUCL = 28.89; investigate ✓ Correct answer

    σ = 1.5/1.128 = 1.330; UCL = 24.9 + 3 × 1.330 = 28.89; 29.0 is above it, a signal.

  3. CUCL = 29.40; no action

    This treats MR̄ as σ (24.9 + 3 × 1.5).

  4. DUCL = 29.80; no action

    This uses the moving-range chart limit D4 × MR̄ = 4.90.

Reference: IASSC Certified Green Belt (ICGB) Syllabus v1.1 (PeopleCert, Jan 2026), §5 Detailed Syllabus, Topic 5.2, item 5.2.2, p.9; IASSC Examination Reference Document V1.4.2a, Control Charts – Table of Constants, p.28

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