SEO, as a marketing strategy, can often generate a good return. However, as the search engines are not paid for the traffic they send from organic search, the algorithms used can and do change, there are no guarantees of continued referrals. Due to this lack of guarantees and certainty, SEO is often compared to traditional Public Relations (PR), with PPC advertising closer to traditional advertising. A business that relies heavily on SEO can suffer major losses if the search engines stop sending visitors. Therefore, businesses should not rely on SEO as a single source of business.