Twelve of twelve participants complete your new sign-up flow in a usability session without hesitating once. Two weeks after launch, almost nobody starts it. What had the session actually established?

The principle — a usability session measures whether someone can, and demand is about whether someone would.

Why the key is correct — Cagan's usability risk asks whether users can figure out how to use it Twelve participants completing the flow answers that and stops there. Value asks whether customers will buy it or users will choose to use it and nobody in the session chose anything; they were handed a task.

Why the others are wrong — reading one clean result as two positive answers is the substitution that separating the risks is designed to catch. Reading fluency as desire mistakes a participant following instructions for a customer making a decision. And ruling the product out because one risk looked good hands a single piece of evidence authority over questions it never touched.

Remember this — everyone finishing the task tells you nothing about whether anyone would have started it.

Sources — Marty Cagan on the four big risks, Silicon Valley Product Group.