Why pass/fail data isn’t enough for root-cause analysis
Root cause analysis
A pass/fail result tells you whether a unit met a test criterion. To investigate why it failed, connect that result to the unit’s production, inspection and repair history.
What you’ll learn
- Which records add context to a failed test.
- How to compare passing and failing units.
- What to validate before calling something a root cause.
On this page
What the test result can and can’t tell you
A failed functional test gives you a starting point. It does not, by itself, establish whether the issue began in assembly, a component, the test setup or another part of the process.
The next question is: what differs between comparable units that pass and those that fail?
Connect the records around each unit
Start with reliable unit or batch identifiers and aligned timestamps. Then bring in the records that can answer your investigation question.
| Record | Question it helps answer |
|---|---|
| Test measurements and limits | What failed, and how close was it to the limit? |
| Inspection and production history | What happened to the unit before testing? |
| Component lots and process settings | Which conditions are shared by affected units? |
| Repair and retest history | Was this a first failure or a repeat test? |
Check missing IDs, inconsistent timestamps and duplicate retests before comparing groups.
Worked example · Illustrative
The same functional-test failure keeps returning
Imagine repeated failures on one product revision. Here is how a team could narrow the investigation.
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Define a fair comparison
Compare the same product revision and test version over the same period. Separate first tests from retests.
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Look for shared conditions
Join each unit’s history. Check whether failures cluster around a station, component lot or process setting, compared with passing units.
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Validate the explanation
Inspect the suspected condition and test the hypothesis. Check whether the failure pattern changes after a controlled correction.
A pattern is a lead, not proof of cause. Check alternative explanations before deciding what to change.
What to check on your line
Use this checklist before your next investigation.
