Metrology is the science of measurement.

Measurement is a procedure in which an unknown quantity is compared to a known standard using an accepted and consistent system of units.

Inspection: is a procedure in which a part or product characteristic, such as a dimension, is examined to determine whether or not it conforms to the design specification.

Accuracy: is the degree to which the measured value agrees with the true value of the quantity of the interest.

Precision: is the degree of repeatability in the measurement process.

Systematic errors: are positive or negative deviations from the true value that are consistent from one measurement to the next (affect accuracy).

Random errors: are usually associated with human participation in the measurement process (affect precision).

Mean (m ) is a measure of the central tendency in the distribution (the average).

where N = number of members in the population; x_i = variable of interest.

Standard deviation (s ) is a measure of spread or scatter in the population expressed in the original units.

± 3s of the population = 99.7% (for normal distribution).

Rules for choosing measuring instrument: