Chapter 4
4.2 Design Requirements : Specifying Functions, Behavior, and Attributes
Specifications (specs) – provide a basis for determining a design because such specs become the targets
of the design process against which we ensure our success in performing them.
Presented in 3 Forms:
1. Prescriptive requirements: specify VALUES for attributes of the designed object
2. Procedural requirements: specify PROCEDURES for calculating attributes or behavior.
3. Performance requirements: specify PERFORMANCE LEVELS that must demonstrate successful
functional behavior
Interface performance requirements – how a system works with other systems
4.2.1 Attaching numbers to design requirements
Designer has to translate functions into measurable terms in order to be able to develop and
assess a design
Must determining the range over which a measure is relevant to a design and deciding how
much improvement is worthwhile
Utility Plot- benefit of an incremental or marginal gain in performance can be found; plotted on the
ordinate on a normalized range from 0 to 1, the level of attribute being assessed is show on the abscissa
Law of Diminishing Returns- eventually gains no longer are of interest
Standard S-curve = threshold comes first and the plateau last
Reverse S-curve starts w/saturated
4.2.2 Setting performance levels
Performance requirements require sound enge, reasonable measurements, and clarified
client interests
1. Determine design parameters that reflect the functions or attributes that must be measured and
the units in which those parameters are to be measured
2. Establish the range of interest for each design parameter.
Utility values below a threshold are treated as equals and above saturation plateau’s are
also indistinguishable b/c no useful gains can be achieved
4.2.3 Interface performance requirements
Interface performance requirements are important for large firms b/c trying to
minimize the total time needed to design, test, build, and bring to market new products
by design teams working at the same time
Easy in design, hard in real life
4.2.4 A caution on metrics and requirements
Include functional and behavioral requirements
Metrics achievement of objectives; applied past tense to see if objectives have been achieved
Requirements to scale the performance of functions; must be met in a design; developed for
future application to specify the functional performance that must be achieved in order to have
a successful design
4.2.5 A not on customers’ requirements
Quality function development (QFD)- builds on performance requirements method w/ the goal of
achieving higher-quality products
