Chemistry Reference
In-Depth Information
110
108
106
104
102
UCL
100
Target
98
LCL
96
94
92
1
2
3
4
5
6
7
8
9
10
11
12
Sample Number
FIgure 4.2
Example control chart used to track and trend accuracy results. UCL refers to
the upper control limit, LCL the lower control limit.
4.3.2 P
recISIon
The precision of an analytical method is defined as the closeness of agreement among
individual test results from repeated analyses of a homogeneous sample. Precision is
commonly performed on three different levels: repeatability, intermediate precision,
and reproducibility.
4.3.2.1 repeatability
Repeatability
refers to the ability of the method to generate the same results over
a short time interval under identical conditions (intra-assay precision). It should be
determined from a minimum of nine determinations that cover the specified range of
the procedure (i.e., three concentrations, three repetitions each) or from a minimum
of six determinations at 100% of the test or target concentration. Representative
chromatographic repeatability results are summarized in Table 4.2, where results are
summarized for six replicate injections of the same sample. The 0.10% RSD easily
passes the ≤2% acceptance criterion.
4.3.2.2 Intermediate precision
Intermediate precision refers to the agreement between the results from within-
laboratory variations due to random events that might normally occur during the use
of a method, such as different days, analysts, or equipment. Think in terms of param-
eters that might change, that are normally not written into a method, or are external
to the method. To determine intermediate precision, an experimental design should
be employed so that the effects (if any) of the individual variables can be monitored.
Typical intermediate precision results are shown in Table 4.3. In this study, analysts
from two different laboratories prepared and analyzed six sample preparations from
one batch of samples and two preparations each from two additional batches (all
samples are assumed to be the same concentration); all data from each analyst were
pooled for the summary in Table 4.3. Each analyst prepared his or her own standards
