Realistic evaluation of the precision and accuracy of instrument calibration systems

Abstract

Calibration of instruments and standards is a refined form of measurement. Measurement of some property of a thing is an operation that yields as an end result a number that indicates how much of the property the thing has. Measurement is ordinarily a repeatable operation, so that it is appropriate to regard measurement as a production process, the 'product' being the numbers, i.e . the measurements, that it yields; and to apply to measurement processes in the laboratory the concepts and techniques of statistical process control that have proved so useful in the quality control of industrial production. Viewed thus it becomes evident that a particular measurement operation cannot be regarded as constituting a measurement process unless statistical stability of the type known as a state of statistical control has been attained. In order to determine whether a particular measurement operation is, or is not, in a state of statistical control it is necessary to be definite on what variations of procedure, apparatus, environmental conditions, observers, operators, etc., are allowable in 'repeated applications' of what will be considered to be the same measurement process applied to the measurement of the same quantity under the same conditions. To be realistic, the 'allowable variations' must be of sufficient scope to bracket the circumstances likely to be met in practice. Furthermore, any experimental program that aims to determine the standard deviation of a measurement process as an indication of its precision, must be based on appropriate random sampling of this likely range of circumstances. Ordinarily the accuracy of a measurement process may be characterized by giving (a) the standard deviation of the process and (b) credible bounds to its likely overall systematic error. Determination of credible bounds to the combined effect of recognized potential sources of systematic error always involves some arbitrariness, not only in the placing of reasonable bounds on the systematic error likely to be contributed by each particular assignable cause, but also in the manner in which these individual contributions are combined. Consequently, the 'inaccuracy' of end results of measurement cannot be expressed by 'confidence limits' corresponding to a definite numerical 'confidence level,' except in those rare in stances in which the possible overall systematic error of a final result is negligible in comparison with its imprecision.