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Special Control Chart - EWMA / EWMA Dynamic |
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The Exponentially Weighted Moving Averages (EWMA) chart is based on the values Xi = W.xi + (1 - W).Xi-1. W (0 < W < 1) is a weight specifying how fast is the EWMA response to change in the process mean. When W=1, the chart is equivalent to the X-bar or X-individual chart. Higher W values produce smoother chart which is less sensitive to sudden changes. This module can also produce a dynamical EWMA modification, useful when the controlled process shows long term oscillations or trend so that other chart types cannot be used. Such a situation can be indicated e.g. by significant autocorrelation or time trend , found in the Basic data analysis module. The dynamical EWMA chart tolerates long term process mean or variance changes, but identifies short term, sudden changes. The dynamical EWMA chart is controlled by the A parameter inputted in the dialog panel. The EWMA residual chart is used for the same purpose as the dynamical EWMA chart.
Extended control charts - Pdf manual
Example
Data:
| Emission |
NOx [mg] |
Output [kW] |
Temperature [C] |
| 64.2 |
13.58 |
214.3 |
482.8 |
| 54.9 |
10.03 |
230.7 |
488.4 |
| 77.1 |
7.58 |
230.6 |
490.4 |
| 69.5 |
6.55 |
243.6 |
500.2 |
| 75.3 |
7.88 |
233.1 |
498.2 |
| ... |
... |
... |
... |
Output:
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EWMA Control Chart |
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Task name : |
Sheet3 |
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Selected columns : |
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Y |
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Number of data : |
127 |
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Central line : |
5.804545001 |
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Std. deviation : |
0.895627611 |
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Weight W for EWMA : |
0.5 |
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Weight Alpha for dynamic limits : |
0.25 |
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No of data outside classical limits : |
4 |
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No. of data outside dynamic limits : |
2 |
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No of residuals outside +-3s : |
2 |
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Mean quadratic residual : |
0.138963062 |
Dynamic EWMA Chart is designed for "wandering mean" or autocorrelated processes.
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QC-Expert 
