Do fashion designers know if people wikll wear there clothes?

I am taking a fashion marketing class and on thing we had to do was give out 100 surveys toour target market. Dsigners usually do some kind of survey (muhc more that 100) to see what people like and don't, then stem from that. The key, my teacher said, is to prove to the investor that people like your desgin and inveting in you isn't a risk.