Upper warning limit

Statistical Factors for the Upper Warning Limit and Upper Control Limit... 

Figure 5.12 A control chart showing a consistent pattern outside the upper warning limit after Day 24, indicating that an evaluation of the situation is warrented. It may indicate a component of the system is out of calibration, etc.

Two aspects are important for IQC (1) the analysis of control materials such as reference materials or spiked samples to monitor trueness and (2) replication of analysis to monitor precision. Of high value in IQC are also blank samples and blind samples. Both IQC aspects form a part of statistical control, a tool for monitoring the accuracy of an analytical system. In a control chart, such as a Shewhart control chart, measured values of repeated analyses of a reference material are plotted against the run number. Based on the data in a control chart, a method is defined either as an analytical system under control or as an analytical system out of control. This interpretation is possible by drawing horizontal lines on the chart x(mean value), x + s (SD) and x - s, x + 2s (upper warning limit) and x-2s (lower warning limit), and x + 3s (upper action or control limit) and x- 3s (lower action or control limit). An analytical system is under control if no more than 5% of the measured values exceed the warning limits [2,6, 85]. 

The average range of the data is multiplied by D to give the lower control limit (Dq ooi). lower warning limit (/I(i.(i25). upper warning limit ( >0.975) and upper control limit ( >0.999). Adapted from Oakland (1992). 

An example of a recovery control chart is shown in Figure 4.7. The mean recovery of individual measurements is represented by the centreline. The upper warning limit (UWL) and the lower warning limit (LWL) are calculated as plus/minus two standard deviations (mean recovery + 2s) and correspond to a statistical confidence interval of 95 percent. The upper control limit (UCL) and the lower control limit (LCL) are calculated as plus/minus three standard deviations (mean recovery 3s), and represent a statistical confidence interval of 99 percent. Control limits vary from laboratory to laboratory as they depend on the analytical procedure and the skill of the analysts. 

When the measurement process is under control, that is when any variation in counts is basically random, the counts obtained with the RMs, in 95% of the cases, fall between the lower and upper warning limits and in 99.1% of cases between the lower and upper action limits. When the variation in the counts does not conform to the pattern that might reasonably be produced by chance variation, then it is concluded that the process is out of control. It means that one or more systematic errors have been introduced into the system. Several tests for detecting out of control situations have been developed. The purpose of each test is to detect a particular non-random pattern in the points plotted on the control chart. These tests have been evaluated by Nelson [42,43]. The following criteria are used for interpreting the microhiological control charts [38,42] ... 

Upper action limit Upper warning limit... 

UWL = upper warning limit LWL = lower warning limit UCL = upper control limit LCL = lower control limit... 

LWL and the upper warning limit UWL are defined as two-thirds of the control limits and... 

Figure 5 A Shewhart means chart. The final two points are above the upper warning limit and would trigger suspension of the analysis and investigation of the process.

Upper action limit (3.09a) Upper warning limit (1.96a) Process mean Lower warning limit (1.96a) Lower action limit (3.09a)... 

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