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Standards & Quality
Control
Repeatability and accuracy
Two important measures of instrument quality are repeatability
and accuracy. Repeatability is sometimes called “reproducibili-
ty”, sometimes “precision”. However described, repeatability is
essential in nearly all forms of measurement, and is fundamen-
tal to accuracy. Repeatability is not accuracy; repeatability
underlies accuracy. Repeatability is a measure of a method’s
ability to obtain the same result time after time on the same
sample under the exact same conditions.
Repeatability or precision is usually quantified as the standard
deviation (S.D.) of a set of measurements normally distributed
about a mean: ±1 S.D. about the mean denotes the range of
values within which 68% of the measurements will fall; ±2
S.D., 95%.
By contrast, accuracy is a measure of how close to the “true”
value a method or measuring device is. An instrument can
repeat without being accurate, but one cannot be accurate with-
out repeating well.
Standards and controls
To be accurate, all measurements ultimately must be referred to
highly reliable independent standards. Advanced Osmometers
are calibrated with stable sodium chloride solutions of known
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