The measured findings about design share a practical lesson: some constraints cannot be argued with. A few are properties of human perception and movement, a few are mathematical facts about flows and control, and the rest record what happens when designed systems meet the world. Knowing which kind of constraint you face is most of the skill.
Limits of the user
The interface laws come from mid-century experimental psychology. The Hick–Hyman Law shows decision time rising with the logarithm of the number of choices. Fitts's Law shows the time to reach a target depending on its distance and size, which is why important buttons are large and near. The Doherty Threshold found productivity rising sharply once a system responded in under about 400 milliseconds, fast enough that attention does not wander.
Two later findings concern attitude rather than speed. The Aesthetic-Usability Effect shows attractive interfaces judged easier to use and forgiven their problems. The Paradox of the Active User shows people refusing to read the manual because they want to start, and staying less efficient for years as a result.
Limits of any system
A second set is closer to mathematics than psychology. Little's Law is an identity: the number of items in a system equals the arrival rate times the time each spends there, so a queue cannot shrink unless one of the other two changes. Ashby's Law of Requisite Variety holds that a controller can regulate only as much variety as it can itself express, and the Conant–Ashby Theorem adds that a regulator is only as good as its model of what it regulates. The Bullwhip Effect shows small changes in customer demand swelling into large swings further up a supply chain.
What happens when design meets the world
The remaining findings are about consequences. Merton's Law of Unintended Consequences sets out why purposive action produces effects nobody planned. Gall's Law observes that complex systems that work grew from simple systems that worked, and that complex systems designed from scratch rarely work at all.
Where it applies
The interface laws are daily tools in software and product design, and they apply equally to forms, signage and control panels. The flow laws apply to any operation with a queue: hospitals, call centres, courts and warehouses all obey Little's Law whether their managers know it or not. Requisite variety applies to management itself: a control scheme simpler than the situation it governs will be overwhelmed, which is one reason rigid rulebooks fail in variable environments. Gall's Law is the standard argument for building software, and perhaps institutions, by growing working prototypes rather than specifying the whole in advance.
Where it is argued over
The group is less contested than most, because several entries are mathematics. The exception is the Uncanny Valley, where measured curves vary with the faces chosen and some studies find no dip at all. Gall's Law is an observation drawn from experience rather than a tested claim, and its critics point to large designed systems, such as some national infrastructure, that did work at first try.
Why it is worth knowing
Designers argue about taste constantly, and much of that argument is unavoidable. This group marks the places where argument is pointless: people take longer to choose from more options, a queue obeys arithmetic, and a controller cannot manage what it cannot represent. Knowing the fixed constraints leaves more energy for the questions that are genuinely open.
The entries behind this essay: Evidence in Design & Systems. Every subject has its essays on the essays page.