52
3
Standards and 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 fundamental 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
™
Osmometers are calibrated
with standards consisting of stable sodium chloride solutions of known
freezing points which bracket those of the expected unknowns. These
standards are related to the most fundamental work on freezing
points of dilute solutions published and are traceable to the National
Institute of Standards and Technology (formerly the National Bureau
of Standards). We also use and provide a sodium chloride reference
solution for checking osmometer accuracy at a point close to the
osmolality of normal blood serum.
To Next Page
Loading...
Need help?
General
