Improvedpcharts to monitor process quality
作者:
CESARA. ACOSTA-MEJIA,
期刊:
IIE Transactions
(Taylor Available online 1999)
卷期:
Volume 31,
issue 6
页码: 509-516
ISSN:0740-817X
年代: 1999
DOI:10.1080/07408179908969854
出版商: Taylor & Francis Group
数据来源: Taylor
摘要:
It is a common practice to monitor the fractionpof non-conforming units to detect whether the quality of a process improves or deteriorates. Users commonly assume that the number of non-conforming units in a subgroup is approximately normal, since large subgroup sizes are considered. Ifpis small this approximation might fail even for large subgroup sizes. If in addition, both upper and lower limits are used, the performance of the chart in terms of fast detection may be poor. This means that the chart might not quickly detect the presence of special causes. In this paper the performance of several charts for monitoring increases and decreases inpis analyzed based on their Run Length (RL) distribution. It is shown that replacing the lower control limit by a simple runs rule can result in an increase in the overall chart performance. The concept of RL unbiased performance is introduced. It is found that many commonly usedpcharts and other charts proposed in the literature have RL biased performance. For this reason new control limits that yield an exact (or nearly) RL unbiased chart are proposed.
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