Classification of Characteristics
A system (deliverable or end-item) is “ specified ” or described in terms of a number of
attributes or characteristics, including functional, geometrical, chemical, or physical properties
and processes. Characteristics can be specified or described in terms of numerical
specifications, which often include tolerances of acceptability. In a complex system there are
typically a large number of characteristics defined on drawings and other documents. As a
result of the Pareto principle (which states that, in general, the large majority of problems in
any situation are caused by a relatively small number of sources), the most cost-effective
approach to quality assurance is to attend to the system or component characteristics that
have the most serious impact on quality problems or failures. This does not mean to imply that
other characteristics should be ignored, but rather that limited resources and activities for
inspection and acceptance testing should be directed first at those items classified as most
crucial or problematic. Characteristics are typically classified into four categories: critical , major
, minor , and incidental (or, alternatively, critical , major A , major B , and minor ). The critical
classification is reserved for characteristics where a nonconformance would pose safety risks
or lead to system failure. Quality plans often specify that items with critical characteristics be
subjected to 100 percent inspection. The major classification is for characteristics where
nonconformance would cause the loss of a major function of the deliverable. The minor
classification is for characteristics where nonconformance would lead to small impairment of
function or to problems with manufacturability or serviceability. Characteristi
cs classified as incidental would have minimal effect or relate to relatively unimportant
requirements. The classification assigned to a characteristic is determined by the designer of
the system in collaboration with others such as the designer of the next higher-level system,
designers of interfacing systems, or staff from manufacturing or construction. Together they
analyze the design characteristics regarding safety and other requirements, and classify them
using a set of ground rules. Classification also applies to kinds of nonconformities or defects,
but this should not be confused with the classification of characteristics. In welded structures,
for example, the specified characteristics often include the “ absence of any cracks or of certain
amounts and kinds of impurities ” in the weld metal. A crack (a nonconformity that could lead
to a catastrophic failure) would be classified as “ very serious, ” whereas a small amount of an
impurity in the weld (a nonconformity that would have no effect on the integrity of the
structure) would be classified as “ minor. ” Classification of characteristics serves as a basis for
decisions regarding modifications, waivers, and deviations at all levels of a system. For example,
classification of characteristics in a higher-level system provides guidance to designers of the
lower-level subsystems and components that comprise the system.
Classifying the braking performance of an automobile as critical (e.g., that the
automobile when traveling at 25 miles per hour should be able to stop within 40 feet on dry
pavement) tells the braking system designers that components of the brakes should be
classified critical as well. Failure mode and effect analysis (FMEA), discussed below, sometimes
plays an important role in this classification process. Sometimes the characteristic
classifications are listed in a separate document, although it is more practical to indicate the
classifications directly on drawings and other specifications by means of symbols such as “ C ”
for critical, “ Ma ” for major, “ Mi ” for minor, and so on. Absence of a symbol normally indicates
the lowest priority, although some organizations denote even the lowest classification with a
symbol as well. Only a small percentage of characteristics should be classified as critical. Too
large a number of characteristics classified as critical could be the sign of poor design: if
everything is critical, nothing in particular is critical!
Testing of Prototypes and Models As shown in Table , a goal of building and testing
models is to systematically reduce the risk that the final product will not be satisfactory. Models
enable data to be gathered about systems and subsystems that will as sist in later stages of
development. Experiments with models reveal design shortcomings. Data about stresses and
strains in components, for example, provide information about potential failure modes in the
final product. (Occasionally, the engineers and technologists who design and build models
become attached to them and do not want to see them get broken or damaged, and they
actually resist doing tests on them. Of course, the objective of modeling is not to create perfect
models but to generate data that will result in a high-quality final product.) While early tests on
components to acquire technical data should be supervised by designers and test personnel,
functional tests on full-scale models should involve people who most resemble typical users.
For products that are to be manufactured in quantities, the design should be verified by means
of a qualification test before manufacturing ramp-up. The qualification testing process happens
in the opposite way of the system design process. While typically the design process happens
top-down, starting with the overall system design and cascading down to the design of
individual subsystems and components, qualification testing happens bottom-up, starting with
the testing and qualification of individual components, then of subsystems, and finally of the
full, completed system.
Inspection and Acceptance Testing of the Final Product
Quality control of items produced repetitively includes inspections on the product and
its components throughout the production process. Characteristics classified as critical are
always inspected, whereas minor or incidental characteristics are not. In automobile
production, for example, the braking and steering performance on every vehicle is tested.
When items are produced in large quantities, some of them might be subjected to destructive
tests; when only one or a small batch of an item is produced, the inspection and testing
procedures must involve non-destructive methods such as radiographic, infrared, and
ultrasonic imaging of critical components. For certain critical components produced in high
volume, sampling is commonly used to reduce the cost of inspection. Based on the results of
tests from a few samples, a statistical inference is made about the quality of the entire batch
or process. Obviously, this method is mandatory when the testing destroys the product. To
check the heat treatment of engine blocks, for example, a piece must be cut from a heat-treated
engine block and tested in a laboratory. Testing of samples is common whenever a process
produces a large number of identical products. Although testing of the end product from a
production process does not lie within the realm of project quality management, per se, the
testing procedures and other quality assurance processes to be used during production should
be specified during any development project where the result is a mass-produced item, and
included as project deliverables. Product designers—who have intimate knowledge of the key
characteristics of the product and its components—are well suited to specify the ways those
components should be quality checked once production begins.
