In addition to the clear definition of acceptance criteria for functional requirements, driven by discussions with business stakeholders and the use of scenarios, it is also important to define acceptance criteria for each individual non-functional requirement attached to the functional requirements, to ensure that the operation of the delivered solution achieves the required standard in terms of performance, availability, security, robustness and usability. In particular, definition of acceptance criteria in the tricky area of usability can be made more measurable by applying the acronym PLUME, which stands for measures of: