Statistical Process Control Charts

The V chart has limitations, since it is based on individual readings and is replaced by a system established on subgroups, where the average value of these subgroups together with their range enables the process spread to be established. 

If the process is under statistical control, it will be represented by a normal distribution with a gaussian type curve and from the distribution of the means, the control limits can be calculated. 

There are many types of control charts and initially, the choice depends on whether the data is Attribute, which is evaluated in terms of whether it meets a given requirement or not, e.g. pass/fail, go/no go, or whether the data is Variable, where it is assessed in terms of 

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Fig. 4.12. Statistical process control chart for TXRF measurement systems. The sensitivity of the system can be controlled by daily calibration with an...

Statistical process control charts (such as the x-bar and range charts) plot measurements as a function of time [Grant and Leavenworth (1988)]. With reference to the current day, what part of these charts approximates an enumerative study What part of these charts approximates an analytic study Are the parts different Are the uses different ... 

Figure 10.2 Statistical process control charts for clearings. Top panel runs chart showing clearings as a function of measurement number. Middle panel x-bar chart with dashed upper control limit (UCL) and lower control limit (LCL) solid horizontal line is the grand mean, X. Bottom panel range chart with dashed upper control limit (UCL) solid horizontal line is the average range, r.

Figure 10.3 Statistical process control charts for issues. See Figure 10.2 for details.

Initial inspection of Figure 10.1 showed what appears to be a cyclical pattern in the clearings and issues. This was confirmed by the statistical process control charts in Figures 10.2-10.4. The broad peaks and valleys in Figure 10.1 seem to repeat every 20 to 25 days. At first, this seemed to be a strange number of days for a cycle - a monthly cycle of 30 or 31 days would have made more sense to some of us. However, the student was quick to point out that the average business month has between 21 and 22 days (365.25 calendar days per year) x (5 business days per week) / (7 calendar days per week) = 260.89 business days per year which, when divided by 12 months, is 21.75 or approximately 22 business days per month. 

Develop a metrics system allowing for quantifiable results wherever possible for example, use statistical process control charts for manufacturing processes and correlating manufacturing deviations with consumer complaint trends. 

Tates AA, Louwerse DJ, Smilde AK, Koot GLM, Berndt H, Monitoring a PVC batch process with multivariate statistical process control charts, Industrial and Engineering Chemistry Research, 1999, 38, 4769 1776. 

Statistical process control charts (SPC charts) are used to plot quality parameter points from samples taken at different times during a run. Even if all of the points are within specifications, when they are plotted on a graph you may see quite clearly that there is a trend that in time will result in off-specification material unless an adjustment is made. An upset or out-of-control situation is both vividly revealed and documented by such a chart (see Figure 16-3). 

FIGURE 17.3 Graph showing a statistical process control chart. 

De Thomas etal. [Ill] studied the production of polyurethanes and showed that NIRS can be used successfully to monitor the course of the reaction in real time. Spectral data were obtained with a dispersive instrument, using standard transflectance probes. An MLR model was derived for the quantitative determination of isocyanate concentrations during the urethane polymerization reaction. Model predictions were used to build statistical process control charts and to detect trends along the polymerization reaction. The authors suggested that the integration of NIRS with process control routines could lead to improvements of product quality and consistency, while minimizing reaction time. However, model predictions were not used as feedback information for any sort of correction of the process trajectory. Similar studies were performed by Dallin [112] for prediction of the acid number during the production of polyesters. 

Statistical Process Control (SPC) The use of statistical techniques (such as control charts) to analyze a process and take appropriate action to maintain statistical control and improve process capability. 

Wheeler, D.J. (1985), Keeping Control Charts, Statistical Process Controls, Knoxville, TN. 

The control can be enabled by multivariate statistical process control (MSPC) using process models, control charts and the like. 

The traditional approach to quality control is to generate charts of various kinds to monitor the performance of a production unit. At a superficial level, statistical process control (SPC) and statistical quality control (SQC) [9] are terms used interchangeably to describe traditional... 

Many of the quality improvement goals for implementation of PAT in the pharmaceutical industry have been achieved by companies in other industries, such as automobile production and consumer electronics, as a direct result of adopting principles of quality management. The lineage of modern quality management can be traced to the work of Walter Shewhart, a statistician for Bell Laboratories in the mid-1920s [17]. His observation that statistical analysis of the dimensions of industrial products over time could be used to control the quality of production laid the foundation for modern control charts. Shewhart is considered to be the father of statistical process control (SPC) his work provides the first evidence of the transition from product quality (by inspection) to the concept of quality processes [18,19]. 

Concurrent validation is conducted under a protocol during the course of normal production. The first three production-scale batches must be monitored as comprehensively as possible. The evaluation of the results is used in establishing the acceptance criteria and specifications of subsequent in-process control and final product testing. Some form of concurrent validation, using statistical process control techniques (quality control charting), may be used throughout the product manufacturing life cycle. 

Statistical process control (SPC), also called statistical quality control and process validation (PV), represents two sides of the same coin. SPC comprises the various mathematical tools (histogram, scatter diagram run chart, and control chart) used to monitor a manufacturing process and to keep it within in-process and final product specification limits. Lord Kelvin once said, When you can measure what you are speaking about and express it in numbers, then you know something about it. Such a thought provides the necessary link between the two concepts. Thus, SPC represents the tools to be used, while PV represents the procedural environment in which those tools are used. 

Control charting, with the exception of basic statistical analysis, is probably the most useful statistical technique to analyze retrospective and concurrent process data. Control charting forms the basis of modern statistical process control. 

In industrial plants, large numbers of process variables must be maintained within specified limits in order for the plant to operate properly. Excursions of key variables beyond these limits can have significant consequences for plant safety, the environment, product quality and plant profitability. Statistical process control (SPC), also called statistical quality control (SQC), involves the application of statistical techniques to determine whether a process is operating normally or abnormally. Thus, SPC is a process monitoring technique that relies on quality control charts to monitor measured variables, especially product quality. 

The Shotscope system also maintains and displays statistical process control (SPC) data in a variety of formats, including trend charts, X-bar and R charts, histograms, and scatter diagrams. This information provides molders with the knowledge that their processes are in control, and, should they go out of control, Shotscope can alert to an out-of-control condition and divert suspect-quality parts. Furthermore, because the Shotscope system can measure and archive up to 50 process parameters (such as pressures, temperatures, times, etc.) for every shot monitored and the information archived, the processing fingerprint for any part can be stored and retrieved at any time in the future. This functionality is extremely important to any manufacturer concerned with the potential failure of a molded part in its end-use application (for example, medical devices). 

The intent in this chapter is not to present in great detail the mathematics behind the statistical methods discussed. An excellent reference manual assembled by the Automotive Industry Action Group (AIAG), Fundamental Statistical Process Control, details process control systems, variation, action on special or common causes, process control and capability, process improvement, control charting, and benefits derived from using each of these tools. Reprinted with permission from the Fundamental Statistacal Process Control Reference Manual (Chrysler, Ford, General Motors Supplier uality Requirements Task Force , Measurement Systems Analysis, MSA Second Edition, 1995, ASQC Press. 

Finally, this year, a standard catalyst supplier accreditation procedure is being implemented. Primary emphasis is on the implementation of control charts and statistical process control (SPC) procedures in the manufacture of commercial catalysts in order to improve lot to lot consistencies (3) for purchased catalysts. 

Conventional quality control procedures fall short of current needs to improve the consistency of purchased catalyst quality and are being supplemented by the use of control charts and statistical process control. 

Discrete data point, extracted irom the log file, can be viewed. The data can also be viewed in tabular form and as a size distribution curve. Data can also be integrated over any selected range. A Statistical Process Control (SPC) option enables the file data to be viewed in standard control chart format either as an X or R chart. 

Malvern (Insitec) ECPS2 is designed to monitor and control particle size distributions from 0.5 to 1,500 pm, at concentrations up to 10,000 ppm, directly in pneumatic powder flow streams. Up to one thousand size distribution measurements per second are carried out at flow velocities from static to ultrasonic. Discrete data point, extracted from the log file, can be viewed. The data can also be viewed in tabular form and as a size distribution curve. Data can also be integrated over any selected range. A Statistical Process Control (SPC) option enables the file data to be viewed in standard control chart format either as an X or R chart. Various interface arrangements have been described, [203] ... 

Statistical Quality Control (SQC) or Statistical Process Control (SPC) is an effective method of monitoring a process through control charts that enable the use of objective criteria for distinguishing background variation from events of significance based on statistical techniques. 

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