FMEA in DMAIC: Process FMEA in the Analyze and Improve Phases

Failure mode and effects analysis (FMEA) is a structured, team-based way to ask what can go wrong at each process step, how badly it would hurt the customer, how often its cause occurs, and how likely current controls are to catch it. In DMAIC, a process FMEA prioritizes which causes to attack, checks that the new process does not create new risks, and feeds the control plan.

What the exam expects

FMEA is tested with the risk-analysis tools of Analyze (VI) in the 2022 CSSBB Body of Knowledge. Expect to know the worksheet elements, rate and multiply severity, occurrence and detection into a risk priority number (RPN), explain RPN's weaknesses, recognize Action Priority (AP) from the AIAG-VDA FMEA Handbook (2019), and pick the action that lowers a given rating.

Key concepts

• Chain: function → failure mode (how it fails) → effect (what the customer experiences) → cause (why it happens) → current prevention and detection controls.
• Severity (S) rates the effect, occurrence (O) rates the cause, detection (D) rates how well current controls catch the cause or failure mode before it reaches the customer. Each runs 1 to 10, and a high D means the failure is hard to detect.
• RPN = S × O × D, from 1 to 1,000. Only 120 distinct values are possible, the scales are ordinal, and very different risks can share an RPN. A fixed cutoff such as "act above 100" is arbitrary.
• Action Priority: the AIAG-VDA handbook replaces RPN with a lookup table that gives each S-O-D combination High, Medium or Low priority, weighting severity first, then occurrence, then detection. High: the team must act to improve prevention or detection, or document why current controls are adequate. Medium: should act. Low: could act. AP ranks the need for action, not the risk itself.
• Seven steps in the AIAG-VDA approach: planning and preparation, structure analysis, function analysis, failure analysis, risk analysis, optimization, results documentation.
• What lowers which rating: severity changes only through a design or process change that removes or softens the effect. Occurrence drops when the cause is prevented, for example by error-proofing. Detection improves with better detection controls, which do not stop the failure from happening.

Worked example: RPN vs AP on four rows

A packaging-line PFMEA, rated by a cross-functional team on the AIAG-VDA scales. AP values come from the handbook's lookup table.

Reading it: ranked by RPN, the safety-related seal failure comes last, behind a cosmetic scuff. AP puts it at High: with severity 9 and occurrence 4, even a good detection rating of 3 does not lower the priority. Both methods agree that underfill needs action. The cosmetic row has the third-highest RPN but Low AP: high occurrence and poor detection on a low-severity effect do not outrank harm to a user. Note also that the fix for row 1 attacks occurrence, not detection: more inspection would have left the cause in place.

FMEA across DMAIC

• Analyze: built from the process map and cause-and-effect work, it focuses the team on the causes worth verifying with data.
• Improve: re-rate after actions, and run the FMEA on the new process to catch failure modes the solution introduced.
• Control: high-priority causes and their controls carry into the control plan, and the FMEA is updated whenever the process changes.

Common exam traps

• Reading the detection scale backwards: 10 means almost certainly not detected.
• Claiming that inspection lowers severity or occurrence.
• Treating RPN as a ratio scale: an RPN of 200 is not "twice as risky" as 100.
• Applying a fixed RPN threshold as if it were a rule.
• Rating occurrence for the failure mode in general instead of for the specific cause.
• Mixing up mode, effect and cause in a scenario question.
• Treating the FMEA as finished once the worksheet is filled in.

Check yourself

1. S = 8, O = 3, D = 4: what is the RPN? (96.)
2. Which rating does a 100% vision inspection improve? (Detection only.)
3. What is the only way to lower severity? (Change the design or process so the effect is removed or less serious.)
4. Why can a row with a lower RPN get a higher AP? (AP weights severity first; RPN weights all three equally.)

Related: FMEA: RPN vs action priority · DFMEA vs PFMEA · Root cause analysis · Control plan

More: DMAIC overview · Project charter · VOC & CTQ · SIPOC · MSA in DMAIC · Process capability · Data collection planning · Graphical analysis · Root cause analysis · Multi-vari analysis · Hypothesis testing · Regression · DOE · FMEA · Define tollgate · Change management · Financial benefits · Team dynamics · Improve tollgate · Piloting & scaling · Poka-yoke (BB) · Solution selection · Control plan · SPC in DMAIC · Sustaining gains

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