What Are the Odds of Becoming a Black Belt?

People often ask me to help them assess what their odds of success will be in some new endeavor: What are my odds of succeeding as a full-time blogger? What are my odds of succeeding as an indie game developer? What are my odds of succeeding as an entrepreneur? On the surface it seems intelligent to assess your risks before embarking on a new venture. Unfortunately the way I've seen most people do this is rather silly...