Analysis Techniques For Man-Machine Systems Design

NATO UNCLASSIFIED- 33 AC/243(Panel-8)TR/7Volume 2technique is closely related to several other software development techniques, particularly to Data Flow Diagrams andFinite State Machine algebra. The state machine model underlies several more advanced techniques (Davis. 1988). APetri net model (2.6) can be considered as an alternative to a state machine model, and is specially suited to modellingconcurrent systems (Peterson, 1981). In an "extended" finite state machine, attributes are associated with the inpuLBehaviour graphs used in the RDD approach (2.7) can be reduced to an extended finite state machine. In fact, this is anecessary step in order to generate simulation models and prototypes directly from the specification.DisadvantagesThe technique does not describe clearly complex human-machine interfaces where multiple control actions (e.g.,several key-strokes) may be required to initiate a changeof state. Normally, the diagrams do not show theoutcome of incorrect operator actions: (transition tablescan be checked for completeness, however, therebyidentifying possibly undesirable operator actions). Theyare inherently sequential in nature, and small extensionsof a system can give rise to an exponential increase inthe number of states that have to be considered (Harel etal. 1988). The flat structure of a state transiuon diagramdoes not describe well the "state explosion" that occurswhen concurrent activities and large systems aredescribed.AdvantagesThe technique is used and understood by softwaredevelopers (Wirstad, 1989). The diagrams are good forcommunicating the required operator input informationfor tasks and possible sequences of operator actions,particularly when multiple options must be analysed,e.g., when developing menus. Transition tables permita rapid, complete, analysis of individual operator controlactions and avoid some of the limitations of thediagrams by introducing the possibility of combiningstates using AND/OR logic to create a hierarchy. It hasbeen suggested that the diagrams would be very usefulfor providing a framework for the analysis of data onoperator actions captured by on-line monitoring ofkeystrokes and other control movements (Maguire &Sweeney, 1989).Relative contributionNo data available.ApplicationsThe technique was used for the development of an integrated, digital-data-bus based ship-board communication systemfor the Canadian Forces. It has been used for the development of training requirements for a ship-board system in theRoyal Navy (UK), and a howitzer. an air-defence system. training system and human performance model, in theUSA.Quality assurance considerationsThe technique must be checked for consistency and completeness. The diagram appears best suited to checking forconsistency, the transition table best suited for checking completeness. The analyst should check that all states havebeen defined and that each can be entered and exited.Relationship to system performance requirementsThe output of the technique cannot be related directly to system performance requirements. System performance isexpressed only as a series of system states, and the logic by which those states are initiated.NATO UNCLASSIFIED- 33; -