  In software-as-a-service paradigms software systems are no longer monolithic
pieces of code executing within the boundaries of an organisation, on the
contrary, they are conceived as a dynamically changing collection of services,
collectively executing, in pursuit of a common business goal. An essential
aspect of service selection is determining whether the Quality of Service (QoS)
profile of a service satisfies the QoS requirements of a client.
  In realistic execution environments, such QoS values might be influenced by
external, non-controllable events, making it impossible for the service
provider to guarantee that the values characterised by a QoS profile will be
met, naturally leading to the need of a probabilistic interpretation of QoS
profile.
  In this work we propose: 1) a model for describing probabilistic QoS profiles
based on multivariate continuous probability distributions, 2) a language for
describing probabilistic QoS requirements, and 3) an automatic procedure for
assessing whether a probabilistic QoS profile satisfies a probabilistic QoS
requirement.
