A lot of people do not in fact know how to measure themselves properly for clothes (many people measure the wrong part of their body for the "waist"). Perhaps even more people think they're a certain size, but have never actually measured themselves, they just go by the measurements on clothes they already own, which are frequently "vanity sized" to make people feel better. I don't envy solving this problem between two sets of users: people who actually have the correct measurements and people who believe strongly that they do, but are quite wrong. Imagine how far off you would be if you went by your Old Navy pants: https://flowingdata.com/2010/09/30/advertised-vs-actual-wais...