An iterative interrogative technique used to explore the cause-and-effect relationships underlying a particular problem.
The primary goal of the technique is to determine the root cause of a defect or problem by repeating the question "Why?". Each answer forms the basis of the next question. The "5" in the name derives from an anecdotal observation on the number of iterations needed to resolve the problem.
Problem: Users are not completing the sign-up form.
They abandon the form on the "Create Password" step.
They are getting an error message that their password is not strong enough.
They don't know what the password requirements are.
The password requirements are not visible on the screen.
The requirements were hidden behind a tooltip to keep the UI "clean", but users don't discover it.