Tham khảo tài liệu 'volume 17 - nondestructive evaluation and quality control part 20', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Fig. 17 Second revision control charts for Table 2 data with two more sample deletions samples 1 and 11 both of which exceed UCL in Fig. 16 resulting from workpieces produced prior to machine being properly warmed up. a -chart. b R-chart. Data now have k 16 n 5. Importance of Using Both I and R Control Charts. This example points to the importance of maintaining both . and R control charts and the significance of first focusing attention on the R-chart and establishing its stability. Initially no points fell outside the -chart control limits and one could be led to believe that this indicates that the process mean exhibits good statistical control. However the fact that the R-chart was initially not in control caused the limits on the -chart to be somewhat wider because of two inordinately large R values. Once these special causes of variability were removed the limits on the -chart became narrower and two values now fall outside these new limits. Special causes were present in the . I data but initially were not recognizable because of the excess variability as seen in the R-chart. This example also points strongly to the need to have 25 or more samples before initiating control charts. In this case once special causes were removed only 16 subgroups remained to construct the charts. This is simply not enough data. Importance of Rational Sampling Perhaps the most crucial issue to the successful use of the Shewhart control chart concept is the definition and collection of the samples or subgroups. This section will discuss the concept of rational sampling sample size sampling frequency and sample collection methods and will review some classic misapplications of rational sampling. Also a number of practical examples of subgroup definition and selection will be presented to aid the reader in understanding and implementing this central aspect of the control chart concept. Concept of Rational Sampling. Rational subgroups or samples are collections of individual .
