Systems usability case in stepwise control room validation

We present a Systems Usability Case approach enabling a requirement-based human factors evaluation of complex socio-technical systems. The approach is especially suitable for stepwise verification and validation activities of nuclear power plant control-room systems. Systems Usability Case is based on the Safety Case approach and exploits the Systems Usability construct. Systems Usability Case is a conceptual procedure within which a reasoning process takes place that enables moving from abstract understanding of human performance and usability of complex tools, to concrete proof of a specific system, and finally to interpreting, in elaborated abstract terms, what is learned about the overall acceptability and level of Systems Usability of the targeted system. The paper introduces the Systems Usability Case approach and demonstrates an application of it to real data from a stepwise control room validation. The results suggest that Systems Usability Case has some advantages: it helps to conduct the validation activities in a more systematic fashion; it makes the reasoning process more explicit and transparent; we are able to build a longitudinal view of the progress of the design process; and constructing the Systems Usability Case enables monitoring of the fulfilment of the requirements from the human factors (i.e., Systems Usability) perspective.