Systems in the real world are made up of three different kinds of "parts" - hardware, software, and humans. Humans have always been the "part" of systems that are most difficult to engineer around. We humans are, indeed, complicated in how we work and somewhat unpredictable in how well we do at any given moment. Nonetheless, humans are an integral part of a system and, as such, the engineering process must be able to understand how best we can fit in from the perspective of achieving the overall system's goals.
The MA&D Operation has been changing the way that the systems engineering community integrates human considerations into the design process for almost twenty years. Our perspective - the human as a system component - is both powerful and the right starting point for engineering analysis. Our knowledge of the human component, as represented by the wide variety of models and tools that we use to predict human performance as a function of job and equipment design allow the human issues to be studied with a level of engineering rigor consistent with that of the hardware and software elements. And, we have applied these tools and methods ourselves to a wide range of commercial and military systems engineering processes. The result - a system where the human is not an afterthought but, rather, an integrated component working in concert with the other hardware, software, and human components.