Human factors

HFE / UE is the engineering discipline that designs the human-machine interaction so that the device can be used safely and effectively by the intended user, in the intended use environment, performing the intended task. It is broader than UX or industrial design — it deliberately starts from the assumption that users will make mistakes, that environments will be hostile, and that the device must be designed so that the mistakes that do occur cannot result in serious harm.

The discipline rests on two equivalent international foundations: IEC 62366-1:2015/AMD 1:2020 (the global consensus standard) and FDA's 2016 guidance "Applying Human Factors and Usability Engineering to Medical Devices". They use slightly different vocabulary but describe the same iterative process: define users and use environments, analyse tasks, identify critical tasks, run formative studies, design risk-control measures, then validate with a summative study.

The first artefact is the Use Specification (IEC 62366-1 §5.1). It is the prose answer to four questions for every distinct user group:

• Who is the intended user? (lay patient, caregiver, registered nurse, surgeon, biomedical engineer — each is a distinct group with different training, experience and limitations).
• What is the intended use environment? (home, ambulance, ICU, operating theatre, hospice — each shapes lighting, noise, time pressure, sterile constraints).
• What is the intended medical purpose? (deliver a bolus, monitor an arrhythmia, image a tumour).
• What are the operational principles? (how the device must be set up, used, maintained, decommissioned).

Each user group becomes a separate cohort in formative and summative studies. "Healthcare professional" is rarely a single group — a community pharmacist, an ICU nurse and an interventional cardiologist have profoundly different mental models, and combining them is one of the most common usability-engineering errors.

Task analysis breaks the use of the device into a hierarchy of tasks and subtasks — typically a hierarchical task analysis (HTA) tree. For each task, the team identifies possible use errors (slips, mistakes, lapses, mode confusion) and, for each error, the worst foreseeable consequence. Tasks whose failure could result in serious harm are tagged as critical tasks.

Critical tasks drive the entire downstream programme: they get formative-study attention, risk-control design, and they must be evaluated in the summative HF validation with sufficient sample size to detect rare use errors. They also become the boundary objects between the HF file and the ISO 14971 risk file — every critical-task use error corresponds to at least one hazardous situation in the risk file.

Formative studies are iterative usability tests run during design to find use errors early and inform design changes. They are cheap, fast and frequent — typically 5–10 representative users per group, sometimes more for high-risk devices — and use whatever prototype fidelity is available (paper, foam, beta software, partially functional bench unit).

• Run formative studies on every critical task, often multiple times as the design evolves.
• Include cognitive walk-throughs and heuristic evaluations alongside user testing.
• Capture every use error, close call, complaint and confusion — they are the design's most valuable diagnostic.
• Feed findings into design changes, risk-control updates and the next formative round. "No changes after formative" usually means the formative was run too late or interpreted too charitably.

Summative HF validation is the regulator-facing study that closes the right side of the V-model from a usability perspective. It is the single piece of HF evidence both FDA and NBs look for first. The protocol is rigid:

1. Production-equivalent device, production labelling, production IFU, production training (where applicable).
2. ≥15 representative users per distinct user group (≥30 if the device is intended for more than one group, etc.).
3. Realistic use environment — actual setting or high-fidelity simulation. No prompting, no coaching, no facilitator help on critical tasks.
4. Every critical task is evaluated. Non-critical tasks may be sampled.
5. Each use error and close call is recorded; every participant is interviewed immediately after the session to capture root cause from the user's perspective.
6. Use-error root causes are categorised (perceptual, cognitive, mode confusion, slip, mistake, lapse) and fed into ISO 14971 risk control.
7. Residual use-related risk is documented and benefit-risk acceptance is signed by management with the authority required by ISO 13485 §5.1.

IFUs, quick-reference guides, on-device labels, symbols, and user training are part of the HF design — they communicate to the user during real use. They must be developed under the same usability process: drafted, evaluated formatively, and included in the summative validation as they will be supplied. ISO 15223-1 governs medical-device symbols; ISO 20417 governs information supplied with medical devices; EU MDR Annex I Chapter III sets specific IFU content requirements.

HF doesn't end at launch. Post-market complaint data is the single richest source of real-world use-error evidence, and EU MDR Article 83 / 21 CFR 820.198 require it to be systematically analysed. Patterns of complaint coded as use-related must trigger re-evaluation of the risk file and, where appropriate, design changes, IFU updates or labelling field actions. PMCF / PMS reports are the natural home for this analysis.

• Treating HF as a UX exercise — UX is part of HF but HF is broader and more rigorous.
• Conducting the summative study on engineers or marketing staff rather than representative users.
• Lumping distinct user groups together ("healthcare professional" as one cohort).
• Under-identifying critical tasks — and being unable to defend the downgrade at audit.
• Closing residual use-related risk by IFU warning alone with no attempt at higher-tier controls.
• Treating formative study only as a marketing focus group.
• Skipping summative on the basis that "the design hasn't really changed since 2010" — when the design predates the 2016 FDA expectations and 2015 IEC 62366-1.