A REVISED DOUBLE SAMPLING CONTROL CHART

Standard Shewhart control chart has been widely used, but it is slow in detecting small shift. A number of alternatives have been proposed to improve the sensitivity of control chart, for example by employing warning limits. The double sampling (DS) control chart is aimed at improving the ability to detect any out-of control condition by observing the second sample without any interruption. The DS control chart was firstly proposed by Croasdale (1974). Daudin (1992) proposed DS control chart that utilizes the information from both samples at the second stage. Irianto and Shinozaki (1998) analyzed both DS procedures, and found that Daudin’s procedure is better than Croasdale’s. Instead of minimizing the expected sample size, Irianto and Shinozaki (1998) maximized the power to detect a small shift of mean value. This optimization produced an out-of-control limit at the first sample as high as 3.6 standard deviation. In many applications in manufacturing companies, this high out-of-control limit is meaningless since it has only 0.3 per thousand opportunities of out-of-control condition. This result leads to a revised DS control chart, by neglecting the decision for out-of-control condition at the first sample. This implies to a reduction of limits parameter from three into two, which thus it simplifies the optimization procedure.

کلیدواژه ها:

Double Sampling Control Chart ، Optimization ، Power of Control Chart ، Sample Size