8
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 the
parameter and a representative lot of reagent
(for each parameter) the following standard
deviations were obtained:
PRECISION AND ACCURACY
Parameter
Standard Solution
Standard Deviation
pH
7.0 pH
0.1 pH units
Free Chlorine
1.00 mg/L
0.03 mg/L
Total Chlorine
1.00 mg/L
0.03 mg/L
Cyanuric Acid
20 mg/L
1 mg/L
Iodine
2.5 mg/L
0.1 mg/L
Bromine
2.00 mg/L
0.08 mg/L
Iron LR
0.80 mg/L
0.01 mg/L
