Tham khảo tài liệu 'engineering statistics handbook episode 8 part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | . EWMA Control Charts Choice of weighting factor The parameter A determines the rate at which older data enter into the calculation of the EWMA statistic. A value of A 1 implies that only the most recent measurement influences the EWMA degrades to Shewhart chart . Thus a large value of A 1 gives more weight to recent data and less weight to older data a small value of A gives more weight to older data. The value of A is usually set between and Hunter although this choice is somewhat arbitrary. Lucas and Saccucci 1990 give tables that help the user select A. Variance of EWMA statistic The estimated variance of the EWMA statistic is approximately s2ewma A 2- A s2 when t is not small where s is the standard deviation calculated from the historical data. Definition of control limits for EWMA The center line for the control chart is the target value or EWMA0. The control limits are UCL EWMAo ksewma LCL EWMAo - sewma where the factor k is either set equal 3 or chosen using the Lucas and Saccucci 1990 tables. The data are assumed to be independent and these tables also assume a normal population. As with all control procedures the EWMA procedure depends on a database of measurements that are truly representative of the process. Once the mean value and standard deviation have been calculated from this database the process can enter the monitoring stage provided the process was in control when the data were collected. If not then the usual Phase 1 work would have to be completed first. Example of calculation of parameters for an EWMA control chart To illustrate the construction of an EWMA control chart consider a process with the following parameters calculated from historical data EWMA0 50 s with A chosen to be so that A 2-A .3 and the square root . The control limits are given by UCL 50 3 LCL 50 - 3 http div898 handbook pmc section3 2 of 4 5 1 2006 10 35 00 .
