When we have to memorize a phone number we have just heard for the first time, we instinctively repeat it to ourselves in groups of three or four digits. Split a number into 010, then four digits, then another four, and even eleven digits slip into our heads quickly. If we hear the digits all at once without breaking them up, the first part starts to fade somewhere in the middle.
In his 1956 paper “The Magical Number Seven, Plus or Minus Two,” American cognitive psychologist George A. Miller discussed the fact that there is a limit to how much information a person can hold at one time. The paper became one of the most cited papers in psychology, and in the design field its ideas have come to be called “Miller's Law” and used as a guideline for designing screens and information.
This article looks in turn at what Miller's Law says, the principle of chunking that underlies it, how to apply the law in design, and the common misconceptions and later research surrounding it.
We Remember Only About Seven Things: The Story of Miller's Law
The Mental Workbench Is Narrower Than You Think
Miller's Law is commonly known as the idea that the number of items an average person can hold in short-term memory at once is about seven, or roughly five to nine at most or least. Short-term memory here is like a mental workbench that briefly holds the numbers or words we have just seen.
In his paper, Miller set two observations side by side.
(1) Distinguishing along a single dimension
When people heard a number of sounds that differed only in pitch and had to identify each one, they made almost no mistakes up to five or six sounds, but errors increased beyond that.
(2) Memory span
When people heard numbers, letters or words and then had to repeat them in order, young adults could generally manage around seven items.
It was intriguing that both results hovered around seven, but Miller stated that this was merely a coincidence. Even the phrase “magical number” did not mean that a mysterious number really existed; it was a rhetorical flourish to make the writing more entertaining. Even so, the number seven made it easy to grasp that there is a clear limit to how much human thought can handle at once, and thanks to this it spread widely even beyond psychology.
Grouping Things into Chunks Lets You Hold More
The more important finding in Miller's paper was that the limit of memory is set not by the amount of information but by the number of chunks. A unit that a person recognizes as a single meaning, whether a single number or a single word, is called a “chunk.”
- Chunking Numbers: Splitting an eleven-digit phone number into three groups reduces the number of chunks to remember to three.
- Chunking Words: A word in a language you know is a single chunk, but a word in a language you don't know scatters into many unfamiliar sounds and is much harder to memorize.
- Expertise: What becomes a single chunk depends on the knowledge a person already has.
In the end, people turn information into familiar groups to fit more content onto their narrow workbench. This process is called “chunking.” The fact that chess masters can remember the arrangement of pieces from a real game position at a glance can also be explained by their seeing several pieces as a single familiar pattern.
Applying Miller's Law in Design
Designers have actively brought this principle into screen design, because the more things users have to remember and compare at once, the more mistakes they make and the faster they tire.
(1) Show information in groups
Spacing card numbers in groups of four digits and splitting a long settings screen into groups by topic make it easier to scan at a glance.
(2) Break things into steps
An application form with a lot to fill in is not crammed onto one screen but divided into several steps, reducing the amount to handle at once.
(3) Make things visible instead of relying on memory
If what the user chose on the previous screen is also shown on the next, they don't need to hold it in their head.
(4) Place things to compare side by side
When comparing products or pricing plans, placing items side by side in the same order means users don't have to go back and forth between screens trying to recall what they saw earlier.
This way, users can spend the effort they used to put into memorizing information on the decisions that really matter. Especially for first-time users of a service or people skimming a screen in a hurry, such small considerations can make the difference between finishing a task and giving up halfway.
Common Misconceptions and Later Research
As widely known as Miller's Law is, it is also often misused.
- Menu Limit Myth: The claim that menus or tabs must always be cut to seven or fewer is an overly broad interpretation of the law, because menus that are always visible on screen don't need to be memorized.
- Individual Differences: The size of short-term memory varies with the person, the situation and the content. Some research suggests a span of about seven for digits, six for letters and five for words.
- Phonological Loop: British psychologist Alan Baddeley argued that for spoken material, memory span depends not on the number of items but on the time it takes to say them aloud, roughly 2 seconds' worth. This is why people remember more short words than long ones.
- Magical Number Four: Psychologist Nelson Cowan argued that the working memory of young adults actually holds about four chunks.
That is why designers today say that rather than memorizing seven as the right answer, we should remember the fact itself that human memory is narrow.
More than any single number, Miller's Law teaches an attitude of respecting human limits. Grouping and trimming information so that users don't have to memorize it is the first step toward comfortable design.
