BrandTech Scientific EASYCAL 4.0 - Manual (Page 24)

24 EASYCAL ™ User Guide Version 4.0 · Status: June 2008
EASYCAL
™
User Guide
Coeffi cient of Variation
The coefficient of variation (CV) is a measure of relative dispersion. It is gener-
ally expressed as a percentage and is defined as standard deviation in % related
to the mean volume. It is a measure of random error, i.e. the closeness of values
of repeated measurements.
Accuracy
Accuracy (A) is defined as the difference between the measured mean volume
(V) and the specified value (V
spec.
), related to the specified value in percent. It
is a measure of systematic error, i.e., the closeness of measured mean volume
to the specified value.
Error limit
The term “error limit” in the corresponding standards defines the maximum per-
missible deviation from the specified value.
When batches are tested, the standard deviation will be displayed to indicate the
reproducibility of the results/test method.
Error limit of A and CV
A good estimate for the error limit at nominal volume of a volumetric instrument
can be calculated using the values for A [%] and CV [%].
Error limits calculated in this way show the maximum deviation. Normally the
deviation is lower.
EL
≥
A% + 2CV%
100 %
· V
nominal
7.1 General definitions
The precision of glass volumetric instruments is commonly defined by “Error limits” whereas for liquid handling
instruments the statistical terms “Accuracy [%]” and “Coefficient of Variation [%]” have been established.
7. Basics for calculations
A [%] =
V
_
- V
spec. · 100
V
spec.
CV [%] =
s · 100
V
_
EL ≥ V
measured
- V
spec.
EL
≥ V
measured
- V
spec.
+ 2s