Advanced Instruments 4250, User Manual

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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”,
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 the ability of
a method to obtain the same result time after time on the same
sample.
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 has come. One can repeat
without being accurate, but one cannot be accurate without
repeating well.
Standards and controls
To be accurate, all measurements ultimately must be referred to
highly reliable independent standards. Advanced
®
Cryoscopes
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