[2]: 169, 256  Walter Vincenti ends the work with a general "variation-selection model" for understanding the direction of technological innovation in human history.

This may be why Dr. Michael A. Jackson, author of Structured Design and Problem Frames, once concluded a keynote address to engineers with the statement, "Read Vincenti's book.

"[3] Walter G. Vincenti (commonly pronounced "vin-sen-tee" in the US or "vin-chen-tee" in Italian) (1917–2019) was a Professor Emeritus of Aeronautical and Aerospace Engineering at Stanford University.

[4] In 1987 he was inducted into the National Academy of Engineering, “for pioneering contributions to supersonic aircraft aerodynamics and to fundamental understanding of the physical gas dynamics of hypersonic flow.”[5] His important textbook from the first part of his career is, Introduction to Physical Gas Dynamics (1ed ed 1965, 2nd ed 1975).

One exception is the fifth case study on flush-riveted joints which involved an intimate interplay between design and production.

This case study illustrates there can be a key relationship between human behavior and engineering requirements that can greatly affect the outcomes.

The propeller case study illustrates how engineers develop methods to account for the absence of required scientific theory.

In this case "parameter variation" was used to map-out and survey a subject where no comprehensive scientific theory (in physics) existed.

This case study also illustrates how there are aspects of engineering that cannot adequately be described as science such as the "feel" rivet mechanics developed for how much pressure to apply when completing the aircraft's aluminum stressed-skin structure (see "tacit knowledge" discussion below).

[11] These observations do not constitute an "engineering method" per se but offer a conjecture that they may point the way for further research.

[2]: 102  This process is referred to as "Seven Interactive Elements of Engineering Learning" and includes: The boldface from the original text isolates the steps in a subject-neutral manner.

[2]: 249 Vincenti concludes that our blindness to the vast potential in variations of design does not imply a random or unpremeditated search.

Likewise, engineers proceed in design “blindly” in the sense that “the outcome is not completely foreseeable” thus the “best” potential variations are in some degree invisible.

He notes, “from the outside or in retrospect, the entire process tends to seem more ordered and intentional—less blind—than it usually is.”[2]: 246 However, Vincenti uses the differences between the Wright brothers and the French to show there is a range in how we manage blindness to variations.

Since, “the French were not inclined toward theoretical analysis, variations could be selected for retention and refinement only by trails in flight.”[2]: 244 [emphasis added]  For the Wrights, advancement of basic principles in theory via analysis lent to precise shortcuts to direct trials making the French process appear more exploratory in retrospect.

Thus, the process of selection is aided by 1) theoretical analysis and 2) experiments (in, say, wind tunnels) in place of direct trial of actual (“overt”) versions in the environment.

Thus the temptation to see a net decrease in blindesss “stems from an illusion.” The variation-selection process can create as much blindness as it reduces; just ask “talented engineers who struggle to advance a mature technology like present-day aeronautics…”[2]: 249 The second factor on uncertainty in the whole variation-selection model is “unsureness” in the process of selection.

Both vicarious and overt trials suffer from unsureness which adds complication to the variation selection model.

[2]: 249 Blindness and unsureness characterize the difficult or arduous nature of technology evolution in the variation-selection model.