When implementing any
statistical method that relies on data, it is important to
be sure that the systems that collect that data are both
accurate and precise. A set of procedures, often referred to
as "Gage Studies", are widely used to assess the quality of
measurement systems. STATGRAPHICS Centurion provides
1. Determining the
repeatability and reproducibility
of a variable measurement system.
2. Assessing the
linearity and accuracy of a
variable measurement system.
3. Analyzing the
properties of an attribute
Variable Measurement Systems - Repeatability and
For measurement systems
that result in quantitative measurements such as weight,
concentration, or strength, it is important to determine the
magnitude of any error in the resulting measurements. If the
error is large, it may be impossible to determine whether or
not an individual sample is within spec. In addition,
designed experiments rely on the ability to separate real
effects of making changes from the background noise and
could be sabotaged by an inadequate measurement system.
measurement error, it is common to separate the error into
repeatability (error due to the instrument or measurement
procedure) and reproducibility (error due to the appraiser).
STATGRAPHICS implements the procedures suggested by the AIAG
(Automotive Industry Action Group), including the average
and range method, ANOVA method (with and without
interaction), and the range method (for short
Variable Measurement Systems -
Linearity and Accuracy
While repeatability and
reproducibility studies concentrate on the variability or
precision of a measurement system, Linearity and Accuracy
studies quantify the bias. In these studies, multiple
measurements are made on reference samples and an equation
is constructed for the bias of the measurements.
Attribute Measurement Systems
When the results of a
measurement system are PASS or FAIL rather than a
quantitative value, special procedures are necessary.
STATGRAPHICS Centurion provides three procedures outlined by
the AIAG to deal with such systems: the risk analysis
method, the signal theory method, and the
analytic method. In the risk analysis method, multiple
appraisers measure samples with known characteristics.
Statistics are calculated based on how often the appraisers
correctly characterize each sample and how frequently they
agree with themselves and each other.