In an open, service oriented environment where multiple providers offering functionally identical services exist, it is desirable that service consumers can make a decision of which service to use based on quality of service (QoS). While considerable effort has been made to develop models and techniques to assist consumers in QoS assessment, the majority of existing approaches tend to ignore consumers? expectation on QoS and consider subjective user ratings only in assessment. In this paper, we propose an expectation-based approach which uses delivered values instead of ratings as the basis for QoS assessment. We provide a conceptual overview of our approach, compare it qualitatively with existing methods, and empirically evaluate its performance.