The Design of Design: Essays from a Computer Scientist

2. How Engineers Think of Design—The Rational Model

A Rational Model of the design process

• Goal

• Desiderata

• Utility function

• Constraints, especially budget (perhaps not $ cost)

• Design tree of decisions

UNTIL (“good enough”) or (time runs out)

DO another design (to improve utility function)

UNTIL design is complete

WHILE design remains feasible,

make another design decision

END WHILE

Backtrack up design tree

Explore a path not searched before

END UNTIL

END DO

Take best design

END UNTIL

... [F]or the theory of design is that general theory of search ... through large combinatorial spaces.

HERBERT SIMON [1969], THE SCIENCESOF THE ARTIFICIAL, 54

The Model

Engineers seem to have a clear, if usually implicit, model of the process of design. ...