BS ISO 7870-3:2020 pdf download.Control charts Part 3: Acceptance control charts.
4 Symbols and abbreviated terms NOTE The ISO/IEC Directives makes it necessary to depart from common SPC usage in respect to the differentiation between abbreviated terms and symbols. An abbreviated term and its symbol can differ in appearance in two ways: by font and by layout. To distinguish between abbreviated terms and symbols, abbreviated terms are given in Arial upright and symbols in Times New Roman or Greek italics, as applicable. Whereas abbreviated terms can contain multiple letters, symbols consist only of a single letter. For example, the conventional abbreviation of acceptable process limit, APL, is valid but its symbol in equations becomes ApL: The reason for this is to avoid misinterpretation of compound letters as an indication of multiplication.
5 Description of acceptance control chart practice
In the pursuit of an acceptable product or service, there often is room for some latitude in the ability to centre a process around its target level. The contribution to overall variation of such location factors is additional to the inherent random variability of individual elements around a given process level. In most cases, some shifts in process level must be expected and can be tolerated. These shifts usually result from an assignable cause that cannot be eliminated because of engineering or economic considerations. They often enter the system at infrequent or irregular intervals, but can rarely be treated as random components of variance.
There are several seemingly different approaches to treating these location factors contributing variation beyond that of inherent variability. At one extreme is the approach in which all variability that results in deviations from the target value must be minimized. Supporters of such an approach seek to improve the capability to maintain a process within tighter tolerance limits so that there is greater potential for process or product quality improvement.
At the other extreme is the approach that if a high level of process capability has been achieved, it is not only uneconomic and wasteful of resources, but it can also be counterproductive to try to improve the capability of the process. This often is the result of the introduction of pressures which encourage "tampering" with the process (over-control) by people qualified to work on control aspects but not product or process quality improvement programmes.
6 Acceptance control of a process
6.1 Plotting the chart The sample average value of the quality characteristic is plotted on acceptance control charts in the following way. A point is plotted on the chart for each sample with an identification number (numerical order, time order, etc.) on the horizontal scale, and the corresponding sample average on the vertical scale.
6.2 Interpreting the chart When the plotted point falls above the upper acceptance control limit ACLu or below the lower acceptance control limit ACLL, the process shall be considered non-acceptable. If a plotted point is close to the control line, the numerical values shall be used to make the decision.BS ISO 7870-3 pdf download.