consequently, have proposed new trust metrics. The simple, holistic approach to transitivity is characteristic to social networks (FOAF, Advogato). It follows everyday intuition and assumes that trust and trustworthiness apply to the whole person, regardless of the particular trust scope or context. If one can be trusted as a friend, one can be also trusted to recommend or endorse another friend. Therefore, transitivity is semantically valid without any constraints, and is a natural consequence of this approach. The more thorough approach distinguishes between different scopes/contexts of trust, and does not allow for transitivity between contexts that are semantically incompatible or inappropriate. A contextual approach may, for instance, distinguish between trust in a particular competence, trust in honesty, trust in the ability to formulate a valid opinion, or trust in the ability to provide reliable advice about other sources of information. A contextual approach is often used in trust-based service composition. The understanding that trust is contextual (has a scope) is a foundation of a collaborative filtering. Operations For a formal trust metric to be useful, it should define a set of operations over values of trust in such way that the result of those operations produce values of trust. Usually at least two elementary operators are considered: fusion that provides a quasi-additive functionality, allowing to consolidate trust values coming from several sources; discounting that provides a quasi-multiplicative functionality, allowing the advice/trust opinion provided by a source to be discounted as a function of the trust in the source, which is the principle for transitive trust computation. The exact semantics of both operators are specific to the metric. Even within one representation, there is still a possibility for a variety of semantic interpretations. For example, for the representation as the logic for uncertain probabilities, trust fusion operations can be interpreted by applying different rules (Cumulative fusion, averaging fusion, constraint fusion (Dempster's rule), Yager's modified Dempster's rule, Inagaki's unified combination rule, Zhang's centre combination rule, Dubois and Prade's disjunctive consensus rule etc.). Each interpretations leads to different results, depending on the assumptions for trust fusion in the particular situatation to be modelled. See for detailed discussions. Scalability The growing size of networks of trust make scalability another desired property, meaning that it is computationally feasible to calculate the metric for large networks. Scalability usually puts two requirements of the metric: The elementary operation (e.g. fusion or discount) is computationally feasible, e.g. that