Empirical design principles

When to use

• Deciding spacing, grouping, hierarchy or emphasis on a page, before it is built.
• Reviewing a layout where a principle is being cited as the reason it works.
• Someone proposes a rule of thumb — "seven items max", "reduce the options" — and it needs checking against what the underlying work actually says.

When NOT to use

• Reaching for a usability heuristic rather than an empirical principle. Nielsen's ten, Shneiderman's eight and the rest are not here, deliberately. They make a different kind of claim, so they fail differently: a heuristic is accumulated judgement organised for inspection, and it fails by missing a problem. A principle here predicts, and fails by predicting wrongly. Applying one is an act of judgement you defend; applying the other produces a number you test. That absence is declared, not covered — see usability-heuristics.
• Reaching for Norman's constraints, forcing functions, or natural mapping. Neither skill covers them. That one is a real gap, not a declared absence: cite the edition and page yourself — DOET rev. ed. 2013 ch. 4 for constraints and forcing functions, ch. 1 and 3 for mapping — and do not read either file's silence as clearance. The two gulfs are covered, in composing-a-screen.
• Picking token values. Contrast, type scale, spacing steps and control heights are owned by the system, not by the findings here — see Cross-references.
• Judging craft quality. Whether something is elite is a separate question from whether its arrangement is well-formed.

They are descriptive, not prescriptive

Every principle here describes what perception or behaviour does under stated conditions. None instructs you what to build. They are ceteris paribus claims — true when nothing else varies — and an interface is precisely the case where everything else varies. Palmer, who introduced common region, says so himself: he calls them "principles" or "factors" rather than laws because "they lack the quantitative structure of standard scientific 'laws'."

So use them forward, never backward. Forward: predict what a layout will do, then build it and check. Backward: ship the layout, then name a principle that sounds like what you did. The second is the dominant failure and it is invisible, because a post-hoc citation is always available for any arrangement — a card grid with 12/16/20px spacing gets defended as "grouping by proximity" when applied forward the same principle predicts it fails. Used before, a principle produces a number you can test; used after, only a name for what already exists.

The one that changes how you space things

Proximity works on ratios, and is scale-invariant. Grouping strength follows relative distance, not absolute distance — the same layout scaled up groups identically. Below roughly a 1.5 ratio between competing gaps, no grouping reliably dominates.

That refutes the common framing. "8px means related, 24px means separate" works because it is 3:1, not because of the numbers, and it silently stops working the moment a neighbouring group uses a different base. A spacing scale that multiplies across breakpoints preserves grouping; one that adds a fixed offset ("+8px on desktop") compresses every ratio toward 1 and dissolves the structure it was meant to protect.

Check ratios, not pixels. 24 against 32 is 1.33 — near-bistable, and readers will parse it differently on different visits.

Two that hold, inside a stated scope

• Fitts's — movement time rises with distance and falls with target size, logarithmically. It models movement to a target of known location, not finding one, and its edge corollary needs a pointer to pin against, not a finger.
• Jakob's — users arrive with a model built on other sites, so an unfamiliar convention costs errors on first encounter. The measured support is control placement and behaviour only; the visual half is Nielsen's opinion.

Four that do not say what design writing says

Read interaction-laws before citing any of these.

• Hick's Law — "reduce the number of options and nest them into categories" is not supported, and on the law's own maths is backwards: nesting charges an extra intercept at every level, so the interface looks simpler and measures slower.
• Miller's Law — "7±2 items in a menu" is the most repeated misquote in interface design. The paper is about immediate memory for unrelated items and supports no menu limit in any form.
• The aesthetic-usability effect (Kurosu & Kashimura 1995; replicated with a controlled manipulation by Tractinsky, Katz & Ikar 2000) — users rate attractive designs as more usable. It is a claim about perception and tolerance. The study that manipulated real usability found aesthetics did not move actual performance, so it never licenses deferring usability work. It is not Nielsen's heuristic 8, "aesthetic and minimalist design" — different authors, and a density claim about irrelevant information competing with relevant, saying nothing about beauty. Fusing the two is the commonest error on this entry; the heuristic side is in usability-heuristics.
• Von Restorff — a memory effect, measured by delayed recall of lists. Nothing in it addresses click-through, so it cannot justify "make the CTA orange and it will be clicked."

Verification

Before citing a principle in a review, state the prediction it makes and the observation that would falsify it. If neither is available, the citation is decoration and the finding should stand on what was actually observed.

For spacing claims, compute the ratio between competing gaps and quote it.

Cross-references

• usability-heuristics — the judgement half: Nielsen's ten and heuristic evaluation, Norman by edition, Shneiderman's eight, progressive disclosure, recognition over recall.
• reviewing-design-work — the dispatcher that routes here when a finding is being justified by naming a principle.
• visual-polish — the rendered-surface lens that catches two gaps reading identical; it defers to the ratio above for how far apart they then have to be, so do not answer that question from the render alone.
• designing-elite-ui — the craft bar, once the layout is sound.
• spacing-system — the spacing ramp these ratios are expressed in.
• component-sizing-principles — control heights and the 24 CSS px interactive floor.
• apca-contrast — figure/ground separation as a measured contrast obligation rather than an intuition.
• consuming-a-design-system, plus your own project's design-system skill if it has one — this skill settles the arrangement, the system settles the value.

Detail sits beside this file on disk — npx skills add copies references/ wholesale — so read it there, not over the network: gestalt · interaction-laws · convention-and-memory. No blob/main URL is given: that promises a path that never moves.

Sources

The three reference files linked above hold 78 source entries. Verification is partial and the scope is the claim: about 20 were opened and read against the original — those marked [read in full], plus the ones the inline corrections turn on — and the rest are DOI- or URL-resolved only. A resolving DOI shows a work exists, not that it says what the citing line claims. Corrections are recorded inline, including who actually established figure/ground, common region and element connectedness, none of whom is Wertheimer.