280
BASIC PRINCIPLES of MEASUREMENT
RELIABILITY AND VALIDITY OF DATA Fundamentally, any item measure should meet two tests: 1. The item measures what it is intended to measure (i.e., it is valid). 2. A remeasurement would order individual responses in the same way (i.e., it is reliable). The remainder of this section describes techniques and procedures designed to assure that measurement systems produce numbers with these properties. A good measurement system possesses certain properties. First, it should produce a number that is ‘‘close’’ to the actual property being measured, i.e., it should be accurate. Second, if the measurement system is applied repeatedly to the same object, the measurements produced should be close to one another, i.e., it should be repeatable. Third, the measurement system should be able to produce accurate and consistent results over the entire range of concern, i.e., it should be linear. Fourth, the measurement system should produce the same results when used by any properly trained individual, i.e., the results should be reproducible. Finally, when applied to the same items the measurement system should produce the same results in the future as it did in the past, i.e., it should be stable. The remainder of this section is devoted to discussing ways to ascertain these properties for particular measurement systems. In general, the methods and definitions presented here are consistent with those described by the Automotive Industry Action Group (AIAG).
Definitions BiasLThe difference between the average measured value and a reference value is referred to as bias. The reference value is an agreed-upon standard, such as a standard traceable to a national standards body (see below). When applied to attribute inspection, bias refers to the ability of the attribute inspection system to produce agreement on inspection standards. Bias is controlled by calibration, which is the process of comparing measurements to standards. The concept of bias is illustrated in Figure 9.1. RepeatabilityLAIAG defines repeatability as the variation in measurements obtained with one measurement instrument when used several times by one appraiser, while measuring the identical characteristic on the same part. Variation obtained when the measurement system is applied repeatedly under the same conditions is usually caused by conditions inherent in the measurement system.