Customers don't know what they want. There's plenty of good psychology research that shows that people are not able to accurately predict how they would behave in the future. So asking them 'Would you buy my product if it had these three features?' or 'How would you react if we changed our product this way?' is a waste of time. They don't know.
Now a cholera epidemic was sweeping through Southeast Asia and south Asia in the early 1970s so I started medical school and I joined a laboratory to work on this.