The channels

Proximity, similarity, common region, uniform connectedness, Prägnanz, salience, legibility, visibility, Von RestorffVon RestorffMeasured, in 1933, that the odd item in a list is remembered better than the alike ones. The isolation effect bears her name.One reference in this work:1933Über die Wirkung von Bereichsbildungen im SpurenfeldSee in the bibliography →. Nine laws, one mechanism: the eye takes in the whole field at once and the brain organises it before you notice you looked. The tenth is Hick-Hyman, which also needs an eye and does not belong to that mechanism.

For someone using a screen reader, that field does not exist. Content arrives in a queue, one item at a time, in the order it sits in the HTML. Two buttons that “are together” because they share forty pixels are not together anywhere. Proximity is a law about pixels, and there are no pixels there — there is a queue.

That queue has a price, and it has been measured. SharifSharifMeasured Fitts’s law in the same 55 people on two different days, with and without fine motor limitation, and it came out statistically different in the two groups. His too is the count of what it costs to read a chart through a screen reader, and the test of which sounds work best for that reading.3 references in this work:2020The reliability of Fitts’s law as a movement model for people with and without limited fin…2021Understanding screen-reader users’ experiences with online data visualizations2022“What makes sonification user-friendly?”: exploring usability and user-friendliness of son…See in the bibliography → and colleagues put 36 people who use screen readers and 36 who do not through the same task: pulling information out of charts published on the web. The screen-reader users were 61% less accurate and took 211% longer. That is not an estimate of discomfort or a self-report of effort: it is the bill an interface hands to someone arriving through the channel it did not plan for.

What replaces proximity is not a new law. It is markup. A group only exists if it is declared: a list, a region, a labelled field, a section heading. The designer who grouped with spacing made a drawing; the one who grouped with structure made both.

One consequence I had taken for granted did not hold up. The aesthetic-usability effectAesthetic–usability effectDo not refine the aesthetics before validating the concept, the finish hides the defect.Open the law → says a beautiful screen seems to work better than it does, and it seemed safe to conclude that without a screen it would vanish. Sharif and colleagues measured something else in 2022, with ten people and as an exploratory study: screen-reader users rating the pleasantness and trustworthiness of a chart’s sonified responses, and in the averages the sound that pleased most was not the one that came through clearest. The authors themselves record that difference as not significant. That is not evidence that the effect survives without a screen, but a sign that the question makes sense in sound and that nobody has answered it yet.

Fitts’s LawFitts’s lawAn important target is a big target, close to where the hand already is.Open the law → is the best-measured law here, and it is a law about arms. It describes the time of a movement that leaves fast towards the target and then enters a phase of fine corrections until it stops inside it.

BravoBravoRepeated the measurement of Fitts’s law in adults with cerebral palsy, in 1993. That measurement is where the number the page cites comes from.One reference in this work:1993A study of the application of Fitts’ law to selected cerebral palsied adultsSee in the bibliography → et al. (1993) measured six adults with cerebral palsy and six without. In the second group the law held. In the first it held for two of the six, and the authors raise two possible causes: range of movement and spasticity, or the protocol itself, with standardised targets and distances and a visual start signal. They end by saying the law may hold again with individualised distances and an audible signal. Sharif et al. (2020) returned to the question with 55 people and found something finer, and the law still describes, with a slightly worse fit.

And there is the case where it does not apply because there is nothing to apply. Someone navigating by keyboard has no distance: they have a number of tab stops. Someone using a switch has a scan that advances on its own, and the cost is waiting time, not movement. A big, close button helps nobody who does not point.

The Hick-Hyman LawHick–Hyman lawCut what does not change the decision. Splitting into steps does not reduce the cost of choosing, it spreads it.Open the law → predicts that doubling the options costs far less than double the time. The logarithm came from information theory, measured with lamps and keys rather than lists, and the version where someone scans a menu at a glance is our field’s rereading of it. A screen reader does not scan at a glance.

I looked for a study that had measured this and did not find one. So it stays written as it is: this is my prediction, not a laboratory finding. If someone has already measured it, I want to read it.