10. GLOSSARY

Accuracy - closeness to the true or accepted (or nominal) value. (See

inaccuracy) .

Apparent mass vs brass - the mass of norml brass (density 8.4 g/cm3 at 0 DC

with a cubical coefficient of expansion of

0.000054/DC) that produces the same balance reading as

the object under test when the comparison is made at

20 DC in air with a density of 1.2 mg/cm3.

Bias - see systematic error

Calibration - comparison of a measurement standard or instruent with another

standard or instruent to detect, correlate, report, or eliminate

by adjustment any inaccuracy of the compared.

Check standard - an artifact that can be systematically measured to provide a

data base Whereby the performce char~cteristics of a

measurement system may be judged. The check standard must be

sufficiently similar to the items tested to permit valid

inferences to be drawn from measurements on the former to

those on the latter (see tranfer standard).

Correction - that quatity When added algebraically to the nominal value

defines its true value. Thus, if the true value is larger than

the nominal value, the correction required is positive.

Conventions - a system of rules, generally accepted, for use in reporting

measurement differences. The following conventions have been

adopted for use When reporting differences (d) between nominal

values (N) and the actual values (T) of standards (S) and test

items (X):

ST - SN + Cs

XT-~+CX

Likewise, in comparing two values,

X - S - d

If d is positive, X exceeds S.

Density - the ratio of the mass of a portion, of a substance to its volume; Le.

density - mass/volume.

Drainage Time - An arbitrarily specified time used in volumetric calibrations

to permit the drainage of a reproducible amount of liquid from

a volume standard or object calibrated and/or to establish a

"wet down" condition. It is measured after flow cessation

(Which see).

.'

Flow cessation - The moment when a full stream IIbreaks" and becomes a trickle

or drip; imortant when measuring drainage time (which see).

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Inaccuracy - deviation from the true or accepted (or nominal) value.

Inaccuracy may result from both imrecision and bias in the

measurement process,

Limiting mean - The value approached by the average of a sequence of

independent measurements of the same quantity as the number of

measurements included in the set approaches infinity.

Mass - the measure of the inertial property of a body. Qulitatively it can

simly be thought of as the quantity of material in a body.

Measurement Assurance - A quality assurance program for a measurement process Program (MA) that quatifies the total uncertainty of the measurements (both random and systematic components of error)

with respect to national or other designated standards.

Measurement Process - A sequence of operations Whose purose is to assign a

numer(s) that represents how much of a certain property

a given substance or obj ect has.

Offset - The difference between a laboratory's assignent for a transfer

standard an the NBS assigned value.

Precision - the degree of mutul agreement of independent measurements of a

single quatity yielded by repeated applications of a process under

specified conditions. It is quatitatively stated by a precision

measure such as the stand deviation, for example.

Prover - a volumetric standard larger than 50 L in capacity.

Qulity assessment - the procedues and activities utilized to verify that a

quality control system is operating within acceptable

limits, and to evaluate the quality of the

product/measurement produced.

Qulity control - the procedures and activities developed and imlemented to

produce products/measurements of desired quality.

Radom errors - errors tht var in a non-reproducible way (fortuitously)

around the limiting mean. For a large set of measurements, the

errors are distributed evenly above and below the average.

Also, small errors occur more frequently than large ones.

These errora can be treated statistically by use of the laws of

probability .

Standard deviation - a measure of the dispersion of individual members of a

population about the limiting mean of the population. It

is described by the symol, ii, and is mathematically

defined as the aquare root of the arithmetic mean of the

squares of deviations of the various items from the

arithetic mean of the Whole.

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Standard deviation, - also called sample standard deviation (symbol, s), a estimated a measure of the dispersion of individual members of a

samle about the mean of the sample. If the sample is

randomly selected from the population, the sample

standard deviation is an estimate of the population

standard deviation. (see Section 7 for calculations).

Standard deviation, - also called "between" standard deviation, sb' a measure long term of the variability of measurements mad.e at different

times when using the same meaurement procedure.

Standard deviation, - also called "within" standard deviation, sw' a measure

short term measure of the variability of a measurement process

during a short period of time, usually the time necessary

to complete one calibration/measurement using a particular sequence of operations. It is useful in deciding how

the precision of measurement is improved by replication

at a given time.

Statistiçal control - a term that describes the operation of a measurement

system in a stable and statistically predictable manner.

A system so operating will produce measurements randomly

distributed about a limiting mean with a fixed standard

deviation.

Systematic errors - errors that are reproducible and tend to bias a result in

one direction. Their causes can be assigned, at least in

principle, and they can have both constant and variable

components. These errors cannot be treated statistically.

Test measure - a volumetric standard up to imd including 50 L (10 gal) in

capaci ty .

Tolerance - the maimu allowable departure of a standard from its nominal

value.

Traceability - the ability to relate an individual measurement result to

national standards of measurement.

Transfer standard - an NBS calibrated artifact for use in evaluating a

measurement process. It is most useful in evaluating the

offset of a measurement process or reference standards.

True mass - the mass of a body. The adjective "true" is frequently used with

the word "mass" to clearly differentiate from the apparent mass of

a body.

Uncertainty - allowance assigned to a measured value to include two maj or

components of error: bias and random error.

10.3