Shewhart plot

A pool of plasma is composed by mixing residual amounts of plasma left over from analyses, thereby ensuring the anonymity of the pool sample. Each series of analyses should contain at least one pool plasma sample. The results of 8-10 selected amino acids of the pool are plotted in a Shewhart plot to make deviations from the previously established target values visible. Actions following these deviations will have to be devised by the laboratory staff according to standard quality control rules. 

Computers incorporated into instruments or connected through stand-alone PCs have expanded the capabilities of instrument systems to include those previously available only by connection to the mainframes of LISs. These capabilities include summaries of quality control (production of Shewhart plots), fitting of immunoassay calibration curves using various models (e.g., four parameter log-logistic), and linkage of patient information with specimen identification. 

Precision data can be documented in bar charts or control charts such as Shewhart control charts (see the discussion of internal quality control in Section 8.2.3.5). Bar charts plot %RSD values with their corresponding confidence interval. Control charts plot the individual measurement results and the means of sets of measurements with their confidence level (or with horizontal lines representing limits, see below) as a function of the measurement number and the run number, respectively [15,55,56, 58,72, 85]. 

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]. 

Figure 4.11. Shewhart means plot of the duplicate analysis of a certified reference material, twice per day for 20 days. Each point is the mean of the day s four results. Warning limits (UWL and LWL) are at the global mean 2xs/i/4 and control (action) limits (UCL and LCL) at the global mean 3xs/ /4, where s is the standard deviation of all the data.

Figure 4.15. Shewhart range plot of data in figure 4.11. The range is of the four points taken each day. Dashed lines are control limits and dotted lines are warning limits.

A pool of erythrocyte samples. This is made up of erythrocytes from left-over EDTA-blood of a large number of different patients. Aliquots of 300 pi suspension are stored in BHT-treated Eppendorf vials at -20°C for a maximum period of 1 year. One of these samples is included in each weekly series of analyses. The results of ten selected fatty acids are plotted on an electronic Shewhart chart. 

An example of the most common control chart, the Shewhart chart, is shown in Fig. 8-46. It merely consists of measurements plotted versus sample number with control limits that indicate the range for normal process operation. The plotted data are either an individual measurement x or the sample mean x if more than one sample is measured at each sampling instant. The sample mean for k samples is cal-... 

Shewhart reasoned that by plotting summary statistics from periodically collected... 

By far the most famous implementations of Shewhart s basic logic come where the plotted statistic is either the mean, the range, or, less frequently, the standard deviation. Such charts are commonly known by the names x-bar charts, R charts, and 5 charts, respectively. As a basis of discussion of Shewhart charts, consider the data given in Table 5.4. These... 

Figure 5.10 shows plots of the individual melt indices, means, ranges, and standard deviations from Table 5.4 against shift number. The last three of these are the beginnings of so-called Shewhart x, R, and 5 control charts. 

It is clear that the manual preparation and continual updating of the charts shown in Fig. 2 for a multilevel, multi-analyte quality control system involves a great deal of work. However, it is possible in a multilevel control system to represent all individual values at different levels on one chart which is a variant of the Shewhart mean plot. The difference of an individual value (e.g. from the target mean (x ) is divided by the target standard deviation (sQ and thus the position of the individual value is represented relative to the target mean in standard deviation intervals 1), see Fig. 1. The bias of each value, irrespective of its analyte concentration, is therefore represented on the same standard deviation scale. This is very convenient for manual and computer plotting as complex scaling is avoided. Fig. 4 shows an example of this... 

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