By the 1980s, a number of theorists were calling for an expanded and modified framework that would be more useful to service marketers. The prospect of expanding or modifying the marketing mix for services was a core discussion topic at the inaugural AMA Conference dedicated to Services Marketing in the early 1980s, and built on earlier theoretical works pointing to many important problems and limitations of the 4 Ps model. Taken collectively, the papers presented at that conference indicate that service marketers were thinking about a revision to the general marketing mix based on an understanding that services were fundamentally different from products, and therefore required different tools and strategies. In 1981, Booms and Bitner proposed a model of 7 Ps, comprising the original 4 Ps plus "process, people" and "physical evidence", as being more applicable for services marketing.