6
Precision
is how closely repeated
measurements agree with each other. Precision
is usually expressed as standard deviation (SD).
Accuracy
is defined as the nearness of a test
result to the true value.
Although good precision suggests good
accuracy, precise results can be inaccurate.
The figure explains these definitions.
In a laboratory using a standard solution of
7.0 pH and a representative lot of reagent,
an operator obtained with a single instrument
a standard deviation of 0.1 pH units.
In a laboratory using a standard solution of 1.00 mg/L free and total chlorine and a representative
lot of reagent, an operator obtained with a single instrument a standard deviation of 0.02 mg/L.
PRECISION AND ACCURACY
PRINCIPLE OF OPERATION
Absorption of Light is a typical phenomenon of interaction between electromagnetic radiation and
matter. When a light beam crosses a substance, some of the radiation may be absorbed by
atoms, molecules or crystal lattices.
If pure absorption occurs, the fraction of light absorbed depends both on the optical path length
through the matter and on the
physical
-chemical characteristics of the substance according to the
Lambert-Beer Law:
-log
I
/
I
o
=
ε
λ
c d
or
A
=
ε
λ
c d
Where:
-log
I
/
I
o
=
Absorbance (A)
I
o
=
intensity of incident light beam
I
=
intensity of light beam after absorption
ε
λ
=
molar extinction coefficient at wavelength
λ
c
=
molar concentration of the substance
d
=
optical path through the substance