Service composition is one of the most interesting challenges facing the Service Oriented Architecture (SOA) paradigm. Its accomplishment promises to maximize the information reuse. Service composition mechanism wants to facilitate application development by combinations of existent resources which are exposed as services. This mechanism is associated with the notion of loose coupling. The diminution of dependencies between services is intuitively understood and gives immediate benefits. However, this intuitive understanding hides a lack of formal theories. In this paper, we intend to provide a clear definition of the loose coupling notion. We reuse the concepts introduced by our Composite Service MetaModel to identify its associated metrics. These metrics will allow for the comparison of existent service composition approaches. Moreover, they define some development guidelines for architects.