Miller's Law

The popular version of this law ('humans can hold seven items in working memory') is overstated. Miller's 1956 paper was much more careful than the slogan suggests, and later research has revised the number down — closer to four for items you don't already know.

But the core idea still matters for product design: information grouped into small chunks of 3–5 is much easier to process than the same information in one flat list.

This shapes how you design onboarding screens (never more than four bullets), how you format numbers (card numbers in 4-digit blocks, phone numbers in local conventions, dates in the order users expect in their country), how you build settings menus (group related options under labelled sections rather than one long list), and how you write error messages (one chunk of context, one chunk of action — never a paragraph).

A 16-digit card number formatted as one continuous string — 1234567890123456 — is nearly impossible for a user to verify against a physical card without pointing at the screen and counting.

The same 16 digits formatted as 1234 5678 9012 3456 is four chunks, each within comfortable working memory, and users verify it twice as fast with half the errors.

The same principle shows up everywhere a PM looks once they start noticing: phone number inputs that auto-format as you type; password fields that break complexity requirements into visible chunks ('1 capital ✓, 1 number ✓, 8+ characters ✗'); settings pages grouped into Account / Security / Notifications / Legal rather than one long list of 40 items.