The System of Quality Control Cards

The quality control card [39] was introduced by Shewhart in 1931 and originally used for industrial product control. The principle of a quality control card is illustrated in Fig. 8-10. It is an optical representation of the quality based on 

With the tolerance limits one distinguishes between so-called warning limits, a single crossing of which is still tolerated, and so-called control- or intervention limits, the crossing of which results in immediate action. 

In a so-called pre-period, one or several control sample(s) of every analysis series are analyzed together with the samples. For each control sample a precision check and, if possible, an accuracy check are carried out by comparison with given values. If the analysis quality is acceptable, warning and intervention limits are defined and the quality control card is drawn. In the subsequent control period, the compliance with the defined limits is monitored by measuring further control samples. The results from these control analyses or the statistical quantities derived from them are continuously registered in the quality control card. The abscissa relates to the continuing serial number, whereas control values are registered on the ordinate. 

the quality control card aids in the fast recognition of errors because the given control criteria indicate an out-of control situation for the monitored analytical process. With a quality control card each of the following statistical quantities can be monitored  

There are two important aspects in defining the limits of a quality control card  

An important parameter for evaluating quality control cards and out-of-control situations is the average run length (ARL) which is defined as the average number of registrations in a quality control card for a single out-of-control situation [41]. A high ARL is required should the analytical process run under control. The probability of a false alarm is then very low. On the other hand, a true out-of-control situation should be indicated as quickly as possible, that is, the ARL has to be relatively small. 

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The application of quality control cards in quality control is based on the assumption that results are normally distributed [42]. Therefore, Shewhart developed a quality control card that shows the bell-shaped curve in a 90° turned form (see Figure 9.10). For this a coordinate system is constructed, in which the ordinate represents the unit of the analytical result being subdivided accordingly. On the abscissa, time, serial number, day, or similar units are registered in a chronological series. After a number of N days, or series, the mean value X and the standard deviation s are calculated and the Shewhart control card is drawn. The central line on the card represents the calculated mean value from the preperiod. The warning and control limits at S 2s and S 3s are plotted with the aid of the standard deviation. 

A description is given of a system of quality assurance, developed by the West German Association of the Adhesives Industry, which takes into account the requirements of its various customers. Based on DIN/ISO 9004, it analyses the possible defects and their results and derives statistical process control data and quality cards based on this analysis. 12 members of the quality assurance association are listed. 

Each series of analyses has one control plasma (a mix of anonymized plasmas), which is also being used for the AAA approach. The observed values of ten individual amino acids are displayed in a Shewhart-card and thus constitute the internal quality control system. 

Whitehead (W6) described an IBM-870 system for the recording of laboratory data on punch cards based on an earlier description by Peacock et al. (PI). The primary purpose of this system was to improve the presentation of individual laboratory reports, but the cards were subsequently sorted and the data on them subjected to statistical analysis, some of which revealed hitherto unsuspected sources of error as a result of variation in the technical performance of individual members of staff. Whitehead was among the first to apply cusum techniques to clinical chemistry laboratory data for control purposes, but his off-line system was not able to provide quality control information of immediate value to the laboratory. 

Batch Analyzers. The American Monitor Programachem 1040 does one test at a time on up to 89 samples at up to 15 results per minute. A prepunched program card automatically sets virtually all of the system variables for each method on insertion into the instrument card-reader. A second-generation instrument, the KDA, was shown in 1975. This provides an integrated system from request slip to report form, with a design heavily dependent on the dedicated minicomputer. Another feature offered is graphics, which allows an oscilloscopic display of calibration curves, kinetic reaction-curves, quality-control points, etc. 

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