Prepare for the ASQ Certified Reliability Engineer (CRE) exam by checking the model and assumptions before calculating. Use the current five-domain map, compare series and parallel mission reliability, and see how surviving test units contribute to an estimate.
Who qualifies, and which CRE outline applies?
CRE requires eight years of relevant full-time paid experience, including three in a decision-making position. The current linked Body of Knowledge is the 2025 five-domain outline.
Decision-making means authority over projects or processes and responsibility for outcomes, without necessarily holding a manager title. One qualifying education waiver can reduce the eight-year total: technical or trade diploma one year, associate degree two, bachelor’s four, or master’s/doctorate five. ASQ lists the three-year decision-making requirement separately.
Check your documentation and any prior-certification provisions against ASQ’s current application requirements. This is the general reliability-engineering credential; work in welding does not create a separate CRE exam outline.
Sources: Certified Reliability Engineer (CRE) Body of Knowledge (2025-CRE-BoK.pdf) — Domains I-V with scored question counts; cognitive levels appendix; Certified Reliability Engineer (CRE) Certification | ASQ — Requirements, exam details, policies, fees, CBT dates
What should you review in the five domains?
The official domain counts total 150 scored questions. Use the full outline for individual tasks and their highest assessed cognitive levels.
The counts indicate exam emphasis, not a prescribed study timetable. Include risk, planning and lifecycle work alongside mathematical models.
| Domain | Scored questions | A useful review task |
|---|---|---|
| I. Reliability Fundamentals | 29 | Review reliability plans, economics, ethics and supplier topics. |
| II. Risk Management | 25 | Review failure modes and effects analysis, fault trees and risk controls. |
| III. Probability and Statistics for Reliability | 35 | Distinguish distributions, probability questions, estimates and censored data. |
| IV. Reliability Planning, Testing, and Modeling | 35 | Review test design, reliability models, demonstration and accelerated testing. |
| V. Lifecycle Reliability | 26 | Connect design, maintainability and failure reporting to lifecycle decisions. |
Practise series and active-parallel mission reliability
With independent component failures, a series model requires both components to survive. An active-parallel model requires at least one to survive.
NIST gives the corresponding product formulas. The original exercise fixes the mission and structure; it does not infer either from a hardware label.
Worked exercise
For this fictional exercise, nonrepairable components A and B both start a fixed 100-hour mission working. Their failures are independent. Their probabilities of surviving that same mission are 0.95 and 0.90. Calculate system reliability for two separate configurations: (a) series, where both must operate throughout the mission; and (b) active parallel, where at least one must operate. Assume no additional common-cause or switching failures.
Show answer
(a) Series: R = 0.95 x 0.90 = 0.855. (b) Active parallel: R = 1 - (1 - 0.95)(1 - 0.90) = 1 - (0.05 x 0.10) = 0.995.
Series success requires both independent components to survive, giving 0.855. Parallel failure requires both to fail, with probability 0.05 × 0.10 = 0.005; success is therefore 0.995. These are probabilities for the stated mission, not mean times, availability or actual weld acceptance. Shared failure causes would require a suitable dependency model; they cannot always be represented by simply adding one independent series block.
Sources: NIST/SEMATECH e-Handbook 8.1.8.2 Series model — Series model assumptions and formulas; NIST/SEMATECH e-Handbook 8.1.8.3 Parallel or redundant model — Parallel model assumptions and CDF product formula
Practise using surviving units in a life-test estimate
A unit still operating when observation stops contributes its observed test time. It is right-censored: its failure time is known only to exceed that observation time.
For the exponential model, NIST’s maximum-likelihood estimate of the failure rate is observed failures divided by total unit test time. Its reciprocal estimates mean time to failure (MTTF). The example states this model rather than claiming the data establish it.
Worked exercise
For this fictional exercise, six independent nonrepairable units have identical exponential lifetimes with an unknown constant failure rate. All start working at time zero and are observed continuously until failure or the fixed 1,000-hour end, without repair or replacement. Failures occur at 300, 500 and 900 hours; the other three survive to 1,000 hours. Calculate total time on test, the maximum-likelihood MTTF estimate and the estimated reliability for a 500-hour mission. Use estimated failure rate = failures / total unit-hours and R(t) = exp(-rate × t).
Show answer
Total time on test = 300 + 500 + 900 + 3 × 1,000 = 4,700 unit-hours. Estimated failure rate = 3/4,700 per hour. Estimated MTTF = 4,700/3 ≈ 1,566.67 hours. Estimated R(500) = exp[-(3/4,700) × 500] ≈ 0.727.
Only three failures enter the failure count; all six units contribute observed time. The three survivors are not failures at 1,000 hours. This is a point estimate conditional on the stated exponential model, not a known population parameter or a guaranteed service life. NIST cautions that maximum-likelihood estimates based on few failures can be imprecise and biased. The spread of three observed failure times alone neither verifies nor disproves the assumed constant failure rate.
Sources: Certified Reliability Engineer (CRE) Body of Knowledge (2025-CRE-BoK.pdf) — Domains I-V with scored question counts; cognitive levels appendix; 8.4.1.2. Maximum likelihood estimation — https://www.itl.nist.gov/div898/handbook/apr/section4/apr412.htm; 8.1.6.1. Exponential — https://www.itl.nist.gov/div898/handbook/apr/section1/apr161.htm
How can you turn practice errors into a study plan?
Map each missed or guessed answer to the current Body of Knowledge task. Identify whether you need to revisit the concept, calculation, evidence or reference lookup.
Choose preparation materials by their coverage of the current outline. Compare an existing book’s contents with that outline before deciding what needs supplementing. Adapt your study time to the gaps you find rather than adopting a fixed preparation period.
To assess an older book, compare its contents with each task in the current five-domain outline. Keep useful explanations and supplement identified gaps rather than assuming a book is complete or unusable solely from its date.
- Before a calculation, state the mission, success condition, repair assumptions, dependence and units.
- Separate supplied population probabilities from estimates calculated using sample data.
- For test questions, identify what was observed, what was censored and which model is assumed.
- For risk and maintenance topics, review the decision, evidence and constraints; a single fitted parameter does not dictate a real maintenance interval.
Sources: Certified Reliability Engineer (CRE) Body of Knowledge (2025-CRE-BoK.pdf) — Domains I-V with scored question counts; cognitive levels appendix; Certified Reliability Engineer (CRE) Certification | ASQ — Requirements, exam details, policies, fees, CBT dates; ASQ Certification Candidate Handbook — Reference materials, calculators, scratch paper; ASQ Exam Results FAQs — Results timing, 550/750 scaled passing score, retake policy
What should you expect on the computer-based exam?
The CRE computer-based exam contains 165 questions: 150 scored and 15 unidentified, unscored pretest items. You have 258 minutes to answer them; the appointment is 270 minutes.
ASQ uses a scaled passing score of 550 out of a possible 750. This is not a raw percentage or a fixed published number of correct answers. A practice-test percentage cannot be converted into the official scaled result.
Check the current registration page and candidate handbook for the language, delivery mode, application dates and fee that apply to your booking. Paper-based forms can differ from the computer-based figures above.
Sources: Certified Reliability Engineer (CRE) Body of Knowledge (2025-CRE-BoK.pdf) — Domains I-V with scored question counts; cognitive levels appendix; Certified Reliability Engineer (CRE) Certification | ASQ — Requirements, exam details, policies, fees, CBT dates; ASQ Certification Candidate Handbook — Reference materials, calculators, scratch paper; ASQ Exam Results FAQs — Results timing, 550/750 scaled passing score, retake policy
What does open book allow?
Use securely bound references and notes that remain bound during the exam. Electronic reference copies and collections of questions and answers are prohibited by the candidate handbook.
Prepare any permitted tabs before entry. If a reference contains practice questions or refresher quizzes, follow the handbook’s requirement to remove or obscure them. Studying with a question bank does not make it a permitted exam reference.
Confirm calculator and scratch-work rules for your delivery mode. A test-center appointment supplies scratch paper; remote testing uses the software Scratchpad. Do not apply the test-center paper rules to a remote exam.
Sources: ASQ Certification Candidate Handbook — Reference materials, calculators, scratch paper
Sources
Exam facts checked against ASQ's CRE page, CRE Body of Knowledge, candidate handbook, and exam results FAQ:
- Certified Reliability Engineer (CRE) Body of Knowledge (2025-CRE-BoK.pdf)
- Certified Reliability Engineer (CRE) Certification | ASQ
- ASQ Certification Candidate Handbook
- ASQ Exam Results FAQs
- ASQ Recertification
- NIST/SEMATECH e-Handbook 8.1.8.2 Series model
- NIST/SEMATECH e-Handbook 8.1.8.3 Parallel or redundant model
- 8.4.1.2. Maximum likelihood estimation
- 8.1.6.1. Exponential