
EWMA CONTROL CHARTName:
For the EWMA control chart, we plot
where p is a fraction between 0 and 1 and \( \bar{x_t} \) is the subgroup average at time t. That is, we plot a weighted average of the current subgroup mean with the previous weighted average. The starting value of y is set to the overall mean. The closer the value of p is to 1, the less prior data affects the current estimate. The control limits are computed as
where \( \bar{X} \) is the overall mean and \( \hat{\sigma} \) is an estimate of the process standard deviation. In some cases, there may be historical data or engineering considerations that determine the control limits. You can set your own control limits by entering the commands:
LET USL = <value> LET LSL = <value> where TARGET is the desired target value and USL and LSL are the desired upper and lower control limits. You can control the appearance of this chart by setting the switches for the LINE, CHARACTER, SPIKE, and BAR commands appropriately. Specifically,
For example, to draw the EWMA values as a solid line and an X, the reference line and the Dataplot calculated control limits as dotted lines, and no user specified control limit, enter the commands:
CHARACTER X BLANK BLANK BLANK BLANK BLANK BLANK
where <y> is a response variable; <group> is a subgroup identifier variable; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax is used for the most common case where the data is divided into subgroups.
where <y> is a response variable; and where the <SUBSET/EXCEPT/FOR qualification> is optional. This syntax is used for the case of ungrouped data.
EWMA CONTROL CHART Y EWMA CONTROL CHART Y1 X SUBSET X > 1
WEIGHT is a synonym for the parameter P. That is, you can enter
to specify the weighting parameter.
LABEL CASE ASIS TITLE CASE ASIS Y1LABEL Wafer Thickness X1LABEL Day LEGEND 1 Statistical Process Control LEGEND 2 EWMA Control Chart . SKIP 25 READ CCXBAR.DAT Y X . CHARACTERS CONTROL CHART CHARACTER FILL ON LINES CONTROL CHART . LET P = 0.5 EWMA CONTROL CHART Y X
 
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Date created: 06/05/2001 