In Step 3: Function Analysis of the AIAG-VDA 7-Step FMEA approach, teams must define not only functions but also measurable requirements.
👉 Requirements answer the question:
“How do we know if the function is being performed correctly?”
If requirements are vague, failures cannot be identified precisely, leading to weak FMEAs. Measurable requirements make FMEAs objective, testable, and auditable.
What are Measurable Requirements in FMEA? #
A measurable requirement is a specific, quantifiable criterion that defines how well a function must perform.
đź“Ś Characteristics of good requirements:
- Specific – Clearly states the expected outcome.
- Quantifiable – Includes numerical limits, tolerances, or ranges.
- Verifiable – Can be checked by test, inspection, or calculation.
Why Measurable Requirements are Important #
- Clarity: Everyone in the team interprets requirements the same way.
- Failure Identification: Failures = deviation from requirements → impossible without measurable targets.
- Compliance: Supports customer-specific requirements (CSRs), ISO 9001, and IATF 16949.
- Traceability: Links functions to test methods and control plans.
Examples – Vague vs Measurable Requirements #
| Function | Vague Requirement ❌ | Measurable Requirement ✅ |
|---|---|---|
| Motor provides rotation | “Motor should run smoothly” | “Motor must run at 2000 ± 50 RPM at 12V” |
| Seatbelt restrains passenger | “Seatbelt must be strong” | “Seatbelt must withstand 10 kN force for 5 sec” |
| Weld joins two sheets | “Weld should hold well” | “Weld must withstand ≥ 5 kN shear load” |
| Painting applies coating | “Surface must look good” | “Coating thickness 30 ± 5 microns” |
| ECU monitors signals | “Software should be reliable” | “ECU must detect abnormal signal within 20 ms” |
đź“Ś The difference: vague requirements are subjective; measurable ones are objective and testable.
Steps to Write Measurable Requirements #
Step 1: Start with the Function
Identify what the system, subsystem, or process should do.
- Example: “Apply torque to bolt.”
Step 2: Define Performance Criteria
Translate the function into quantifiable values.
- Example: “Torque = 100 ± 5 Nm.”
Step 3: Add Tolerances & Conditions
Include ranges, environmental conditions, or time limits.
- Example: “Torque = 100 ± 5 Nm at 25°C ± 5°C.”
Step 4: Link to Verification Method
Decide how the requirement will be tested or verified.
- Example: Torque verified with calibrated digital torque wrench.
Case Study – PFMEA for Bolting Process #
- Function: Secure suspension arm with bolt.
- Requirement (Measurable): Apply torque of 100 ± 5 Nm within 3 seconds using power tool.
- Verification: Torque monitoring system with automatic recording.
- Failure Mode: Under-torque (<95 Nm) or Over-torque (>105 Nm).
👉 Because the requirement was measurable, the PFMEA team could clearly identify and analyze risks.
Best Practices for Writing Measurable Requirements #
- Use engineering specifications (RPM, torque, force, temperature, time, dimensions).
- Include units and tolerances (e.g., ±, ≥, ≤).
- Always link requirements to customer specifications or standards.
- In PFMEA, align requirements with control plan characteristics (special characteristics → measurable limits).
- Validate requirements through design verification (DV) or process validation (PV) tests.
Common Mistakes to Avoid #
- Using subjective terms like “good,” “strong,” or “reliable.”
- Missing tolerances (e.g., “Torque = 100 Nm” → is 101 Nm acceptable?).
- Ignoring environmental conditions (temperature, humidity, vibration).
- Defining requirements without linking them to verification methods.
Key Takeaways #
- Functions tell what must be done. Requirements tell how well it must be done.
- Measurable requirements are specific, quantifiable, and verifiable.
- Without measurable requirements, failure analysis becomes weak and incomplete.
- Strong requirements = better risk identification, prevention, and compliance.
Next Lesson #
👉 Continue with Lesson 3.4.3: Function Net & Chain of Functions
