Sustainable competitive advantage is a definite sign of a quality business. This trait ensures that the stock earns returns over the long-term and at the same time helps an investor to preserve his capital. Much of the investment returns that accrue to investors from the quality factor depends on the ability of the business to persist with its supernormal returns on capital. However, the excess returns can persist only if the business is able to keep competition at bay, which is a factor of the sustainability of the competitive advantage of the business. Gauging a business' barriers to entry and barriers to exit are the prerequisites to evaluating its real valuation. exit barriers are extremely relevant when analyzing the ability of a business to persist with supernormal returns. While entry barriers determine the ability of competition to make inroads in the business, exit barriers determine the competitive structure that persists among the incumbents within the industry. Essentially, exit barriers dictate what happens once you are inside the castle. If you find that life gets miserable, exit barriers determine your ability to leave in search of greener pastures. An investor who is able to measure these two barriers accurately is on his way to investment genius. I found an interesting article on the same thought, here's the link: http://multi-act.com/evaluating-sustainable-competitive-advantages-entry-exit-barriers/