Working memory: what a page asks you to carry

In 1956 the psychologist George Miller wrote one of the most quoted papers in psychology, “The magical number seven, plus or minus two”.

It is often read as a rule that people can remember seven things. Miller was careful to say something subtler¹: that memory is limited by the number of meaningful chunks, not the number of letters or digits, and that the sevens turning up in his data were probably “a pernicious, Pythagorean coincidence”. In 2001 Nelson Cowan reviewed the evidence and put the real limit, when people cannot repeat or group what they are holding, at about four chunks².

Chunking is why a phone number is written in groups. “4821 7730 2291” is three things to hold; “482177302291” is twelve.

The capacity you have at your best is not the capacity you have when it matters.

Acute stress reduces working memory, according to a review of 51 studies³, and so does losing a night’s sleep. It declines steadily with age from our twenties, even as what we know keeps growing. An estimated four in five children with ADHD show a significant working-memory difficulty.

Researchers have also argued that money worries take up mental room: in a much-discussed 2013 study, poorer shoppers did worse on reasoning tests after thinking about a large bill. Later attempts to repeat it have had mixed results, so it is better taken as a question than a settled fact. Either way, the person filling in your form may be anxious, tired or interrupted, often all three.

Each time a site makes someone carry something from one screen to another, it is spending their working memory.

A verification code sent by text, an order number from an email, a price from the page before, a choice made three steps ago: some people will drop them, and blame themselves. The accessibility guidelines now say so plainly. A sign-in that makes people retype a code by hand, with no way to paste it, does not comply⁸, and a process must not ask for the same information twice⁹.

Menus are the opposite case. People do not memorise a menu they can see; they recognise what they want in it. That is why Miller’s number says nothing about how long a menu may be.

This one runs here, on this page: it measures you, not a design. Use what it shows you when you review a prototype or a live page with the other tools.