Biomedical Engineering Reference
In-Depth Information
and factors) for using sample measurements to reach a disposition decision.
Some simple examples include:
Averaging a specified number of results and comparing averages to limits
Comparing all individual results to limits
Comparing both sample mean and sample standard deviation to limits
In all of the above cases, the sample size has an impact on the inference that can be
drawn on the related population (batch). More sophisticated schemas can include
tiered testing or simultaneous evaluation of mean and variance. In all cases, the deci-
sion-making capability of any particular schema can be evaluated through an OCC.
Finally but importantly, given the context of this chapter, PCA is one possible
technique that can be used to compare data in the form of distributions, multivariately.
The purpose of including PCA is to compare the ability of the multivariate classifi-
cation achieved through this technique with the allocations obtained through the use
of univariate metrics in either MSA- or OCC-based techniques.
8.4.2
MSA Approach: Definitions and Basic Concepts
MSA can be used to characterize measurements, to analyze total variance, and to sepa-
rate product- and measurement-related sources of variance. Fundamentally, this
approach views obtaining a measurement as a process, the “product” of which is the
measurement value obtained. The information obtained through MSA therefore speaks
to the precision of the measurement. In turn, this allows evaluations of the adequacy of
measurement and measurement system relative to their intended purpose.
It is useful to start a discussion of MSA and the approach used in the evaluation
of EDA with some definition of terms. In their manual describing MSA, the
Automotive Industry Action Group (AIAG) has published the following definitions
pertinent for the current discussion [ 5 ]:
Measurement is defined as “the assignment of numbers [or values] to material
things to represent the relations among them with respect to particular properties.”
This definition was first given by Eisenhart [ 6 ]. The process of assigning the
numbers is defined as the measurement process, and the value assigned is defined
as the measurement value.
Gage is any device used to obtain measurements, frequently used to refer specifically
to the devices used on the shop floor, and includes go/no-go devices (also, see [ 7 ]).
Measurement system is the collection of instruments or gages, standards, opera-
tions, methods, fixtures, software, personnel, environment, and assumptions
used to quantify a unit of measure or fix assessment to the feature characteristic
being measured; the complete process used to obtain measurements.
If we consider a typical pharmaceutical QC situation, where multiple units from a
large batch are sampled and tested, then the variance of these multiple measurements
Search WWH ::




Custom Search