Lay & patient-facing design
Everything else in this system is written for a trained reader whose judgement catches the interface's mistakes. This page is for the case where that reader does not exist — where the person using the software is the patient, has no training, no supervision, and nobody to ask.
Overview
Voice & tone defines a clinical register: precise, hedged correctly, written for someone who knows what a reference range is. That register is right for every clinician-facing surface here and actively unsafe on a patient-facing one, where its precision reads as reassurance and its hedging reads as vagueness.
This is a second register, not a simplified version of the first. Simplifying clinical prose produces something that is still clinical prose with shorter words in it. What is needed is different content: what happened, what it means for you, and what to do — in that order, in sentences a tired person can act on.
Clinical software is designed for a reader who can tell when the screen is wrong. A clinician seeing an implausible value checks the electrodes. A patient seeing an implausible value believes it — and may act on it. Removing the expert reader removes the last error-detection layer in the entire system, and everything on this page is an attempt to put something back in its place.
Say what to do
A patient-facing screen that only conveys information has done half a job. The action is the part that changes an outcome.
Assume the worst moment
The interface is read at 3 a.m., one-handed, by someone frightened or physiologically impaired. That is the design case, not the edge case.
Comprehension is measurable
Whether a person understood is testable, and until it is tested it is an assumption rather than a claim.
Two registers
| Clinical register | Lay register | |
|---|---|---|
| Reader | Can detect an implausible output | Cannot. Believes the screen |
| Primary content | The finding, its provenance and its limits | What it means for this person, and what to do |
| Hedging | Precise and necessary — "suspected", "cannot be excluded" | Read as evasion. State the uncertainty as an instruction instead: "this needs checking by a person" |
| Numbers | Values with units and reference ranges | Values with what they mean; ranges only where the person acts on them |
| Vocabulary | Clinical terms, unglossed | Everyday words; clinical term added in brackets where the person will meet it again |
| Failure | Ambiguity a clinician resolves from context | Confident misunderstanding, acted on alone |
- Everyday word first, clinical term in brackets — "a blocked artery (occlusion)" — but only where the person will encounter the term again. Otherwise leave it out.
- Never a euphemism. "Your results were not entirely normal" is kinder and unusable. "Your results show something that needs checking" is both.
- Avoid negations, and never stack them. "Do not delay if you are not improving" takes two passes to parse and is read at speed exactly once.
- Second person, active voice. "Contact your GP this week", not "contact with the practice should be made".
- One idea per sentence. Subordinate clauses are where instructions get lost.
- Do not soften an instruction into a suggestion. "You may wish to consider" is not an instruction and will not be followed.
Numbers a person can use
Numeracy is unevenly distributed and does not correlate with intelligence or education in the way designers assume. Probabilities, ratios and decimals are all understood poorly by a large proportion of adults, and the ones who misread them do not know they have.
| Instead of | Use | Why |
|---|---|---|
| "a 12% chance" | "about 12 in every 100 people" | Natural frequencies are understood far more reliably than percentages. |
| "a 1 in 8 risk" | "about 12 in every 100" | Keep the denominator constant across every risk on the screen, or they cannot be compared. |
| "your risk is doubled" | "from 2 in 100 to 4 in 100" | Relative change without a baseline is uninterpretable and reliably alarming. |
| "0.5 units" | "half a unit (0.5)" | Decimals are a dose-error source. A misread decimal point is a tenfold error. |
| "within normal limits" | "in the usual range" | "Normal" is heard as a judgement about the person. |
- Never a bare percentage for a personal risk. Always the natural frequency.
- Constant denominators. "1 in 8" beside "12 in 100" cannot be compared by anyone under stress.
- Both directions of a risk where it matters: how many people this happens to and how many it does not.
- Units spelled out at least once — see International design, where units are the primary hazard, and note that a patient cannot sanity-check an order of magnitude.
- No trailing zeros, no naked decimals.
.5is a tenfold error waiting to happen; write0.5.
The worst moment is the design case
Hypoglycaemia is a cognitive impairment. So is pain, panic, exhaustion, and being woken from deep sleep. A patient-facing medical device is disproportionately used at the moments its user is least able to read carefully, reason about numbers, or recover from a mis-tap — and those are precisely the moments its output matters most.
Every readability, target-size and action-count rule in the rest of this system was written for a tired clinician. Here they are insufficient, and the correct response is not to make the interface prettier but to make the urgent path shorter, larger and harder to get wrong.
- The urgent action is the largest thing on the screen and reachable without reading anything else.
- One-handed reach. A phone held in the non-dominant hand while the dominant one is doing something else is the normal case, not an accessibility accommodation.
- No timed interactions. Nothing dismisses itself, nothing expires, nothing requires a response within a window (WCAG 2.2 SC 2.2.1).
- Destructive and therapy-affecting actions need a deliberate, distinct gesture — never adjacent to a frequently-tapped control. See Therapy recommendation.
- Recovery from every mis-tap. Undo, or a confirmation that states the consequence in the lay register.
- No reliance on remembering a previous screen. Each screen carries what is needed to act on it.
Comprehension is a measurement
Every rule above is a Reasoned claim in the sense defined on Status & limitations. None of it is evidence that a particular person understood a particular screen, and that is not a gap this system can close on a manufacturer's behalf.
| Measure | How it is obtained |
|---|---|
| Recall | Can the person state, unprompted, what the result said and what they should do? |
| Correct belief | What do they now think was assessed — and, critically, what do they think was excluded? This is where scope failures show up. |
| Intended action | Presented with a change in symptoms, what would they do and how quickly? |
| Error under load | The same tasks with time pressure, interruption, or a simulated impairment. |
| Range of users | Including low literacy, low numeracy, non-native speakers, older adults and people with the condition — not a convenience sample of colleagues. |
Formulaic reading-level scores are a weak proxy and a useful floor: they measure sentence and word length, not whether an instruction was understood. Use them to catch obvious drift, never as evidence of comprehension.
Do's and don'ts
Your results show something that needs checking by a doctor. The clinic will contact you within 5 working days.
What happened, what it means, what happens next — and a named timeframe the person can hold someone to.
Your results were not entirely within normal limits. Follow-up may be indicated.
Two hedges and a passive construction. The person cannot tell whether this is serious, whether anyone is doing anything, or what they should do.
This happens to about 12 in every 100 people who take this medicine. It does not happen to the other 88.
A natural frequency with a constant denominator, and both directions. Comparable against every other risk on the screen.
Incidence of this side effect is 12%, representing a 2.4-fold increase over baseline.
A percentage, a relative multiplier and no baseline. Reliably read as more frightening than it is, by the people least able to check.
If your vision becomes blurred or suddenly worse, contact your optometrist or GP the same week.
Named symptoms, named contact, named urgency. The person can check themselves against it without interpreting anything.
Seek medical attention if you experience any concerning symptoms.
It asks the person to perform the triage the device exists to help with. "Concerning" is exactly the judgement they do not have.
Take half a unit (0.5) now.
Words and numeral together, leading zero present. A misread decimal point cannot produce a tenfold dose.
Administer .5U stat.
A naked decimal, an abbreviated unit and a Latin abbreviation, on a screen read by a frightened person at three in the morning.
Accessibility
- Narrow viewport is the primary case. Patient-facing surfaces are read on phones; 320 px is the design target, not the reflow floor (SC 1.4.10).
- No timed content or auto-dismissal (SC 2.2.1). A message that vanishes was never read by the person who most needed it.
- Targets larger than the 24 px minimum — treat 44 px as the floor here. Tremor, cold hands and impaired coordination are ordinary conditions of use, not edge cases.
- Reading order matches visual order, and the action follows the finding immediately in both.
- Plain language is an accessibility requirement, not only an editorial preference — it is what makes the content usable with a cognitive impairment.
- Never colour alone for urgency, and never an alarm hue for information — see Colour.
- Text, never an image of text (SC 1.4.5). Patient-facing results are exported, printed and posted more often than they are read on screen.
- Translation is not optional, and translated safety copy is re-tested rather than assumed equivalent — see International design.
Clinical safety notes
Trace these in your risk file (ISO 14971) and usability engineering file (IEC 62366-1).
- Action stated alongside every finding. Mitigates: a person who understood the result and did nothing.
- No euphemism or hedged abnormality. Mitigates: a significant result read as reassurance.
- Natural frequencies with constant denominators. Mitigates: misjudged risk driving refusal of, or insistence on, treatment.
- Leading zeros; units spelled out. Mitigates: tenfold dose error from a misread decimal.
- Urgent path largest and reachable without reading. Mitigates: failure to act during physiological or emotional impairment.
- No timed interactions or auto-dismissal. Mitigates: safety-critical content disappearing before it was read.
- Symptom-based, contact-named, urgency-named escalation copy. Mitigates: deterioration the person did not recognise as actionable.
- Comprehension treated as a measured outcome. Mitigates: readability assumed from word length rather than established with real users.
Related
- Voice & tone — the clinical register this one sits beside.
- Therapy recommendation — where a lay instruction changes the body.
- Autonomous result — delivering a finding to the person it is about.
- Unattended operation — the device running while its user sleeps.
- Usability & context of use — why the specified users of a home device are not a variant of the ED ones.
- Status & limitations — comprehension here is unmeasured, and it is named as a gap.