The Pisano blog explaining the Sean Ellis Test:
Sean Ellis test was formed thanks to the experiences gained during the consultation period, and in a very short time, it became a standard throughout the industry. The most indicative question addressed to the clients in this qualitative test is as following:
How would you feel if you could no longer use our product? - Very disappointed - Somewhat disappointed - Not disappointed (it really isn’t that useful)
If the ratio of the answer “very disappointed” is more than %40, then well-done to you; that means you have the product/market fit.