**GLP 1**

**Good Laboratory Practice**

**for the**

**Quality Assurance of the Measurement Process**

Quality Assurance of the Measurement Process means establishing,
documenting, implementing, and maintaining a quality system appropriate
to the laboratory's scope of activities. Having such a system in place
will allow the laboratory to know, within the limits of a measurement
process, that a measurement is valid with respect to its traceability,
accuracy, and precision.

The validity of tests and calibrations should be monitored with quality
control procedures. Statistical techniques are used to record and
monitor charted measurement results to permit the detection of trends.
The metrologists and laboratory management should plan and review the
results from quality assurance monitoring.

Other steps taken to ensure the quality of the measurement process may
include, but are not limited to:

-   the regular use of Standard Reference Materials (SRMs) and /or
    internal quality control using secondary reference materials;

-   participation in interlaboratory comparisons (round robins);

-   test replications with same and/or different methods;

-   recalibration of retained items;

-   correlation of different characteristics of an item; and

-   proper calibration intervals.

Each measurement parameter in the laboratory's scope of activities
should be reviewed and analyzed to determine the validity of the
measurement process.

The standards and the measurement process for each parameter must be in
a state of statistical control. Statistical control means that the
variability of the measurement process is known and stable; when a
process is in statistical control, we can assume that the reported
measurement uncertainties are valid. The National Institute of Standards
and Technology provides technical guidance and support to the State
weights and measures laboratories to develop measurement control
programs that provide measurement assurance. The objective of these
programs is to evaluate the entire measurement process including:

-   procedures;

-   standards;

-   equipment;

-   personnel; and

-   environment.

While other quality assurance programs could meet these objectives, the
control programs developed for measurement assurance greatly increase
the comprehen­siveness of the program.

The State laboratories typically provide measurement services in the
disciplines of mass, volume, and length. Some laboratories provide
services in other measurement areas. Approximately 89 % of their
workload is in mass standards calibration.[^1] Mass calibration demands
the greatest precision and is the first discipline for which a
measurement assurance program was developed and implemented.
Nevertheless, all measurement disciplines should have a measurement
assurance system in place.

The most recent improvement in assuring the quality of each measurement
parameter in the State Laboratories is the incorporation of a Process
Measurement Assurance Program (PMAP).[^2]

A measurement assurance program consists of duplicating the measurement
process by including a check/control standard as surrogate for the test
item. Measurements made throughout the year will show all the conditions
that are likely to affect the measurement process and their combined
effects. Controlled duplication of the process provides for the
realistic evaluation of the variability of the measurement process as
one of the components in the estimation of the measurement uncertainty.

Measurement results that are collected over several years may be
statistically evaluated with current results being compared to results
from previous years. Any observed problems or changes in the measurement
results are investigated and if necessary, corrective action can be
taken. Ongoing monitoring establishes a continuous, comprehensive,
internal measurement assurance program in a State laboratory.

Data from internal measurement assurance programs is compared to the
results of inter-laboratory comparisons (round robins) in which the
laboratory participates as part of the Regional Measurement Assurance
Program (RMAP) groups. (These groups consist of the metrologists of the
State laboratories and their counterparts in private industry in a
geographic region of the country. Six regional measurement assurance
groups operate in the United States and its territories. )

The strength of the measurement assurance approach lies in its
applicability to a wide variety of measurements with sufficient
flexibility to permit each measurement control program to be tailored to
the particular needs of a given measurement area. The sophistication of
the control program depends on the criticality of the measurement.

[^1]: State Laboratory Program Workload Survey, 1999, NIST/OWM & NCWM
    Metrology Subcommittee

[^2]: See NISTIR 6176 and SOP 30.