Tham khảo tài liệu 'statistical process control 5 episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Process control using variables 127 They pointed out that regardless of the ease or difficulty of grouping the data from a particular process the forming of subgroups is an essential step in the investigation of stability and in the setting up of control charts. Furthermore the use of group ranges to estimate process variability is so widely accepted that the mean of subgroup ranges R may be regarded as the central pillar of a standard procedure. Many people follow the standard procedure given on page 116 and achieve great success with their SPC charts. The short-term benefits of the method include fast reliable detection of change which enables early corrective action to be taken. Even greater gains may be achieved in the longer term however if charting is carried out within the context of the process itself to facilitate greater process understanding and reduction in variability. In many processes there is a tendency for observations that are made over a relatively short time period to be more alike than those taken over a longer period. In such instances the additional between group or medium-term variability may be comparable with or greater than the within group or short-term variability. If this extra component of variability is random there may be no obvious way that it can be eliminated and the within group variability will be a poor estimate of the natural random longer term variation of the process. It should not then be used to control the process. Caulcutt and Porter observed many cases in which sampling schemes based on the order of output or production gave unrepresentative estimates of the random variation of the process if R dn was used to calculate Ơ. Use of the standard practice in these cases gave control lines for the mean chart which were too narrow and resulted in the process being over-controlled. Unfortunately not only do many people use bad estimates of the process variability but in many instances sampling regimes are chosen on an .
