The purpose of this kind of testing isn’t to prove anything. Proving things requires quantitative testing, with a large sample size, a clearly defined and rigorously followed test protocol, and lots of data gathering and analysis. Do-it-yourself tests are a qualitative method whose purpose is to improve what you’re building by identifying and fixing usability problems. The process isn’t rigorous at all: You give them tasks to do, you observe, and you learn. The result is actionable insights, not proof.