The correct answer is C. Position lenses . In FAST diagramming, functions are expressed in a disciplined active verb / measurable noun format, and the purpose of the diagram is to show how project, product, or process functions logically relate to each other. The SAVE certification study material identifies FAST as a graphical mapping tool that helps team members understand how functions relate, while preserving the basic function and analyzing supporting functions. ( )

An “all-the-time” function is a supporting or simultaneous function that must occur continuously while the product performs its basic function. FAST construction tests this by asking: when this function is being performed, what other function must also occur? This “when” relationship identifies functions that occur together, not necessarily functions timed by a clock.

For prescription eyeglasses, the lenses must be held in the correct optical position relative to the wearer’s eyes at all times. Therefore, Position lenses is the best all-the-time function. Improve focus and Access vision describe higher-order or basic-path purposes, while Display information is not a normal function of prescription eyeglasses.

References/topics: Function Analysis Phase; FAST Diagramming; All-the-Time Functions; Basic and Secondary Functions; Function Logic.