Ishikawa / Fishbone / PHA — Structuring the First Pass
The one idea
An FMEA is only as complete as its failure-mode list, and that list has to come from somewhere. Ishikawa (fishbone) diagrams and Preliminary Hazard Analysis are how you brainstorm it systematically, category by category, instead of relying on whoever’s in the room to remember everything from experience.
Mechanics
An Ishikawa diagram starts from a defined effect (an observed or feared problem) and branches into standard categories of contributing cause. Adapted for an analytical lab, the categories are usually:
- Method — the procedure itself: parameters, steps, order of operations
- Materials — reagents, standards, reference materials, columns, consumables
- Machine — instrumentation: hardware, software, calibration state
- Manpower — analyst training, technique, fatigue, handoffs
- Environment — temperature, humidity, lighting, vibration, power quality
Preliminary Hazard Analysis (PHA) is a lighter, earlier-stage cousin — a first-pass brainstorm of what could possibly go wrong before a process even exists in detail, often just a simple hazard/cause/effect table, used to scope what a later, more formal risk assessment needs to cover.
Worked example — “unexpected peak in a stability sample”
| Category | Candidate causes brainstormed |
|---|---|
| Method | Insufficient gradient resolution; wrong wavelength selected; integration parameters too aggressive |
| Materials | Column degradation; contaminated mobile phase; reference standard cross-contamination |
| Machine | Detector lamp aging (baseline drift creating false peaks); carryover from a prior injection; autosampler needle wash insufficient |
| Manpower | Sample prep error introducing a degradant precursor; mislabeled vial swapped with another study |
| Environment | Lab temperature excursion affecting sample stability between prep and injection |
This is deliberately a long, unfiltered list — the point of the fishbone pass is coverage, not judgment. Three or four of these branches then become the failure-mode column of a follow-on FMEA: “contaminated mobile phase” becomes a scoreable failure mode with its own severity, occurrence, and detection; “detector lamp aging” becomes another. The fishbone did the brainstorming; the FMEA does the prioritizing.
When to reach for it vs. FMEA directly
Skip straight to FMEA when the failure modes are already well understood from experience — a mature, well-characterized method rarely needs a fresh fishbone pass. Reach for Ishikawa or PHA first when the process or method is new or unfamiliar, or when a cross-functional team is starting from very different mental models of what could go wrong and needs a shared, structured brainstorm before anyone starts scoring anything.
Known weaknesses
- Purely qualitative — a fishbone diagram or PHA table has no scoring or prioritization built in; it can surface a long list of contributing factors without telling you which ones actually matter.
- Coverage depends heavily on who’s in the room; the category headings help structure the brainstorm, but they don’t guarantee completeness the way a systematic top-down decomposition (like FMEA’s step-by-step structure) does.
- It is not, on its own, a complete quality risk management record — it’s the front end that typically feeds into an FMEA or risk ranking and filtering exercise, not a substitute for either.