EWMA Chartoverview how to example data see also
EWMA ChartStat > Control Charts > Time-Weighted Charts > EWMA
A chart of exponentially weighted moving averages. Each EWMA point incorporates information from all the previous subgroups or observations. EWMA charts can be custom tailored to detect any size shift in the process. Because of this, they are often used to monitor in-control processes for detecting small shifts away from the target.
The plot points can be based on either subgroup means or individual observations. When data are in subgroups, the mean of all the observations in each subgroup is calculated. Exponentially weighted moving averages are then formed from these means. By default, the process standard deviation, s, is estimated using a pooled standard deviation. You can also base the estimate on the average of subgroup ranges or subgroup standard deviations, or enter a historical value for s.
When you have individual observations, exponentially weighted moving averages are formed from the individual observations. By default, s is estimated with MR / d2, the average of the moving range divided by an unbiasing constant. Moving ranges are artificial subgroups created from the individual measurements. The moving range is of length 2, since consecutive values have the greatest chance of being alike. You can also estimate s using the median of the moving range, change the length of the moving range, or enter an historical value for s.
For more information, see Control Charts Overview and Time-Weighted Control Charts Overview.
All observations for a chart are in one column: Choose if data are in one or more columns, then enter the columns.
Subgroup sizes: Enter a number or a column of subscripts. If the subgroup sizes do not vary much, you may want to force the control limits to be constant by specifying a fixed subgroup size using EWMA Options > Limits.
Observations for a subgroup are in one row of columns: Choose if subgroups are arranged in rows across several columns, then enter the columns.
Weight for EWMA: Enter the weight to be used in the exponentially weighted moving average. The value specified must be a number between 0 and 1. By changing the weight used and the number of standard deviations for the control limits, you can construct a chart with specific properties. You can choose combinations of these two parameters by using an ARL (Average Run Length) table. See [17] for an extensive table.