Hach DR/850, Procedures Manual, Page: 36 / 460

36
CHEMICAL ANALYSIS INFORMATION
, continued
Accuracy and Precision
Accuracy is the nearness of a test result to the true value. Precision is how
closely repeated measurements agree with each other. Although good
precision suggests good accuracy, precise results can be inaccurate (see
Figure 8 ). The following paragraphs describe how to improve accuracy
and precision of analyses by using Standard Additions.
Figure 8 Precision and Accuracy Illustrated
Standard Additions
Standard Additions is a common technique for checking test results.
Other names are “spiking” and “known additions.” The standard
additions technique can test for interferences, bad reagents, faulty
instruments, and incorrect procedures.
Perform Standard Additions by following the Standard Additions Method
section in the procedure under
Accuracy Check . Follow the detailed
instructions given.
If you get about 100% recovery for each addition, everything is working
right and your results are correct.
If you don’t get about 100% recovery for each addition, a problem exists.
You can tell if you have an interference. Repeat the Standard Additions
using deionized water as your sample. If you get about 100% recovery for
each addition, you have an interference. If you didn’t get good recoveries
with the deionized water, the following checklist may help to find the
problem quickly:
1. Check to see that you are following the procedure exactly:
a) Are you using the proper reagents in the proper order? Are you
using 10-mL reagents with a 10-mL sample or 25-mL reagents
with a 25-mL sample?
b) Are you waiting the necessary time for color to develop?
Not accurate,
not precise
Accurate,
not precise
Precise,
not accurate
Accurate and
precise
Not accurate,
not precise
Accurate,
not precise
Precise,
not accurate
Accurate and
precise
Not accurate,
not precise
Accurate and
precise
Precise,
not accurate
Accurate,
not precise