Quality control histograms

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. 

Figure 1 taken from the petroleum industry shows a quality control chart where the data in the frequency histogram is normally distributed. In this particular control chart, the grand average is 7.08 and is surrounded by + 1, 2, 3 standard deviations rather than range values. 

Deming principles, or other soundly based quality management concepts, cannot be applied without the use of statistical control methods cause-and-effect diagrams, control charts, run (trend) charts, Pareto charts, flow charts, check sheets, histograms. Juran and Gryna are very good on this subject. So also are Cyrus Derman and Sheldon M. Ross in Statistical Aspects of Quality Control. 

The BMS deviation is a measure of the spread of values for c around the mean. A large value of O indicates that wide variations in c occur. The probability that the controlled variable hes between the values of Cl and C9 is given by the area under the distribution between Ci and Cg (histogram). If the histogram follows a normal probabihty distribution, then 99.7 percent of aU observations should lie with 3o of the mean (between the lower and upper control limits). These Emits are used to determine the quality of control. 

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). 

Quality Awareness. The new giobai economy has introduced a competitive way of doing business, industriai manufacturers use advanced quaiity techniques to stay ahead of the competition. These techniques are taught openiy and used by the entire company. Technicians shouid be aware of these quaiity techniques, which inciude fiowcharts, controi charts, statisticai process control, scatter plots, histograms, Pareto charts, run charts, lSO-9000 criteria/certification, and training. 

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