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Standards & Quality
Control
Repeatability and accuracy
Two important measures of quality control are repeatability and
accuracy. Repeatability is sometimes called reproducibility or
precision. Repeatability is a measure of the ability of a method
to obtain the same result time after time on the same sample.
Repeatability is usually quantified as the standard deviation
(S.D.) of a set of mea surements 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 has come. One can repeat
without being accu rate, but one cannot be accurate without
repeat ing well.
Standards and controls
To be accurate, all measurements ultimately must be referred to
highly reliable independent standards. Ad vanced
®
osmometers
are calibrated with standards con sisting of stable sodium chlo-
ride solutions of known freezing points which bracket those of
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