3
Standards & Quality
Control
A. Purpose of quality control
The purpose of quality control is to ensure that the results
obtained through laboratory analysis are correct. All results
obtained should be as close to the truth as the analytical
technology used is able to determine.
B. Repeatability and accuracy
Two important measures of quality control are repeatability
and accuracy. Repeatability is sometimes called reproduc-
ibility 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 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 accu-
rate without repeating well.
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