MENTAL MODEL · M6618

Function Analysis System Technique

SystemsmediumSystems Theory

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Updated 2026-08-09

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INTRODUCTION

English translation pending.

CORE DEFINITION

Function Analysis System Technique (FAST) is a diagramming method developed by Charles Bytheway in 1965 for value engineering. Functions are arranged in a chain and interrogated with two questions: Why moves left toward higher-order functions, and How moves right toward the means of achieving them. When is asked vertically to identify functions that occur simultaneously. Scope lines bound the analysis, with the left line marking the boundary of the study and the right line marking the basic function the system must perform. The technique's premise is that reasoning about functions rather than components reveals redundancy and value gaps that a parts list conceals.

SCAFFOLDING EFFECT

psychology

Reduce cognitive load

- Use why-chaining: keep asking why a function exists until you reach one that needs no justification. - Use how-chaining: list every mechanism that could deliver the function before choosing. - Use scope lines: fix the boundary of the analysis so the diagram does not expand indefinitely.

anchor

Anchor fast decisions

Components describe what a design contains, while functions describe what it accomplishes, and only the second can be evaluated for necessity. Asking why repeatedly converts a stated function into the higher purpose it serves, which exposes functions that exist only out of habit. Asking how enumerates alternative means, which opens the design space beyond the current implementation. Because the chain links means to ends explicitly, any function that no higher purpose justifies, or that has a cheaper equivalent means, becomes visible as a candidate for elimination.

MINIMUM ACTION

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Practice this model in one real situation:

State the object of analysis and list its functions as verb-noun pairs. 2. Ask why each function exists and draw the link to the higher-order function. 3. Ask how each function is achieved and list alternative means. 4. Draw scope lines marking the study boundary and the basic function. 5. Flag functions that no higher purpose justifies or that have cheaper alternatives.

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Source support: Explicit

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en.wikipedia.orghttps://en.wikipedia.org/wiki/Value_engineeringZH · Explicit
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