Permanent, temporary, and situational barriers overlap in need
Aliases: shared functional need · same functional outcome · need overlap
What it is
Captions let someone who cannot hear follow a conversation, and they let someone who has muted the carriage follow it too. Keyboard operation serves people who cannot point steadily, and people whose trackpad has died. Overlapping accessibility needs means that three triggers — permanent, temporary, situational — often ask the interface for the same functional outcome, not three different product requirements. Write the need as “for screen-reader users” and you cover one biography. Write it as “information does not live only in the image” and all three triggers still stand.
Why it happens
An interface answers which channel is missing now, not what caused the gap. Success criteria are written as functional outcomes (text alternative, keyboard operable, pausable) because those outcomes are diagnosis-blind. The second layer sits in the requirements file: diagnostic labels split the overlap into three niche asks that get scheduled apart or implemented as only one of them; a functional outcome collapses them into a single line, and design only has to show that the outcome holds. What overlaps is the need, not fluency — the same caption track is used differently by someone who depends on it and someone who opens it once.
Studying it
Map each accessibility requirement to at least one permanent, one temporary, and one situational trigger, and see whether all three point at the same acceptance condition. In the defect corpus, compare items filed under diagnostic labels with items filed under “missing channel,” and measure how many of the latter can be merged.
Independent variables: whether the requirement sentence uses a diagnostic label or a functional outcome. Dependent variables: count of mergeable requirements; number of trigger kinds covered by one acceptance condition; whether the test plan recruits only one biography.
Do not treat brief simulated restriction as proof that “the need is the same.” Simulation tests channel loss; it does not tell you whether three requirement documents should have been written as one. The object of study here is the requirement and acceptance text, not completion time.
Where it stops holding
Overlap in need does not make test participants interchangeable. People who use assistive technology permanently have strategies and settings; people in a situational bind often have no screen reader installed and no plan to install one. Pain and fluctuating illness also add “can I finish today,” which does not all fold into one success criterion. Comprehension difficulties from language or culture are not this overlap. If an adaptation requires assistive technology the person already owns, the situational trigger no longer coincides automatically — the requirement must say it does not depend on extra equipment.
Applying it
- Write accessibility requirements as functional outcomes: “state is not colour alone,” “submit can be completed from the keyboard,” and put three trigger stories under the same line rather than as three tickets.
- Rewrite persona sentences that use diagnostic labels as missing channels; if a sentence will not rewrite, it is not this requirement.
- Accept the outcome, not three forked checks keyed to “is this person disabled.”
- How to check: sample the requirements file. Every item that names only one diagnosis gets two more trigger sentences. If they will not write, the item is not a functional outcome — send it back.