Step 3 - Perform Function Analysis

Using the Context of Use, the next step in the user/human-centered design process is to translate mission needs into top-level system functions from which a preliminary functional architecture is derived. The functional architecture defines the operations and events that must be performed in order to meet the system requirements--without regard to function allocation to the human or to automation. The function analysis is expressed and documented in the preliminary functional architecture, using functional flow diagrams. These diagrams must show, at a minimum:

• Functional flow & interrelationships - sequencing and interrelationships of functions, including dependencies amongst functions or on external events for initiation, continuation, or completion.

• Information flow - the flow of information required for decisions, function initiation or completion.

• Function hierarchy -

  • Priority - the relative importance of functions;
  • Duration - estimated time required for execution of functions and the minimum, maximum, mean times defined in system requirements;
  • Frequency - expected recurrence of functions, both planned and due to failure to complete properly;
  • Probability of function completion/accuracy - estimated probability of function completion or required accuracy. Defined as maximums, minimums, or means and may be defined from system requirements or from events external to the system. Includes estimates of the effects of failure to properly execute functions.

The first iteration of function analysis is high-level in preparation for STEP 4 in which functions are allocated to the "human-side" or "machine-side" of the system. Steps 3 and 4 are re-iterated until sufficiently detailed to guide system hardware and software architecture development.