Error tolerance, low physical effort, and enough space are body-scale rules

What it is

The door takes forty newtons. Close is a sixteen-pixel cross in a corner. One stray tap empties the form and there is no way back. Mace bundles tolerance for error, low physical effort, and size and space for approach and use: they all meter what a body pays in time and space — wrong presses, fatigue, not reaching. They are read as a bundle because enlarging a button while still requiring a two-second hold, or offering undo while putting the control above wheelchair reach, leaves the bill unpaid.

This is not a widget-level target-size list. Those details come later, per channel. This is universal design’s standing claim that people have volume, miss, and run out of force.

Why it happens

A body is not an ideal pointer. Force has a ceiling; sustained press and repetition write fatigue as error. Joint range and a seated posture turn “visible on the screen” into “the hand cannot arrive.” Missing is the default, not the exception. Error tolerance turns the sequel of a miss from catastrophe into recovery: dangerous actions hard to graze, ordinary actions reversible. Low effort deletes sustained force and precise paths from the contract. Size and space make approach, reach, and grasp (or a hit) independent of height, posture, or a mobility aid.

The second layer is that the three cuts dump cost onto each other. Large targets packed together raise misses, so size steals error tolerance. Hiding a dangerous action behind a long-press “foolproof” fails low effort and locks out tremor and switch users. Putting a control at reachable height but behind a path gesture gives space without effort or tolerance. Acceptance therefore cannot measure only one cut. In software the translations are: undo, a single click, target and spacing together, main actions not in a hover-only corner — corresponding to a body, not to “make the UI look airy.”

Studying it

Use body-conditioned tasks, not opinion scores. Make one deliberate error and see whether the system collapses or recovers. Complete the same task one-handed, seated, or with reduced force, and record sustained-press demands, repetition counts, and reach. On physical products measure opening force, handle height, knee clearance; on screens measure target size and spacing, whether undo exists, whether the main action requires a path or a long-press.

Independent variables: target size and spacing, reversibility, demand for sustained force or a path, operator posture (seated / one-handed / at reach limit). Dependent variables: recoverability after a miss, repetitions and duration needed to finish, whether every main action can be contacted in that posture.

Log failures against Mace’s fifth through seventh principles separately. Do not collapse them into one “body-friendly” score. Easy completion by young people in a lab does not transfer to fatigue or a seated posture.

Where it stops holding

Safety-critical actions sometimes must cost effort or take two steps, so a brush does not fire them; error tolerance then means “dangerous actions are hard, with a reversible prelude,” not a light tap on an emergency cut-off. A very small screen cannot hold large targets and all content at once — show one group of actions at a time, rather than sixteen tiny crosses in one corner. In strength sports or musical performance, effort is the content; the principles govern the threshold of reaching the content, not rewriting performance as a single click. Remote operation has no knee clearance; size-and-space becomes hit-target and focus visibility, not physical centimetres.

Applying it

• Destructive actions get undo or a real confirmation. Ordinary actions complete on a single click; long-press must not be the only foolproofing.
• Put main actions in the default visible region, with target and spacing together, not in a hover-only corner.
• Do not require a sustained hold, a pinch, or a precise path to submit. If those exist, sit a click alternative beside them.
• How to check: miss on purpose once and see whether the data can come back; finish one-handed, not leaning on the desk; sit (or put the screen at reach limit) and see whether main actions remain in range. If any of the three still depends on “push harder / aim better / start the form again,” the bundle has not yet landed on that body.

Related

• Same group: J1.06.1 Equitable use and flexibility in use have to be designed as one problem · J1.06.2 Simple and intuitive still fails if the information cannot be perceived
• Nearby: J3.04 Target size minimum · J3.09 Accidental activation and undo · A8.16 Tremor
• Search terms: tolerance for error · low physical effort · size and space for approach