CUSUM Chart - Options - Plan/Type
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Stat > Control Charts > Time-weighted Charts >  CUSUM > CUSUM Options > Plan/Type

Allows you to choose and define the CUSUM.

Dialog box items

Type of CUSUM

One-sided (LCL, UCL): Click to generate two one-sided CUSUMs. Use the upper CUSUM to detect upward shifts in the level of the process and the lower CUSUM to detect downward shifts. This chart uses an UCL and an LCL to determine when a process is out-of-control. See [26] and [27] for a discussion of one-sided CUSUMs.

Use FIR: Check to use the FIR (Fast Initial Response) method to initialize the one-sided CUSUM.

Number of standard deviations: Enter the number of standard deviations above and below the center line. Normally they are initialized at 0, but if the process is out of control at startup, the CUSUMs will not detect the situation for several subgroups. This method has been shown by [16] to reduce the number of subgroups needed to detect problems at startup.

Reset after each signal: Check to reset the CUSUMs to their initial values whenever an out-of-control signal is generated. When a process goes out of control, an attempt should be made to find and eliminate the cause of the problem. If the problem has been corrected, the CUSUMs should be reset to their initial values.

Two-sided (V-mask): Click to generate a single two-sided CUSUM that uses a V-mask to determine when a process is out-of-control. See [16] and [30] for a discussion of the V-mask chart.

Center on subgroup: Enter a subgroup number on which to center the V-mask. If no value is entered, Minitab centers the V-mask on the last subgroup.

CUSUM Plan: These values are often selected from ARL (Average Run Length) tables. See [16] and [17].

h: Type a value greater than 0 for h. For one-sided CUSUMs, h is the number of standard deviations between the center line and the control limits. For two-sided CUSUMs (V-mask), Minitab calculates the half-width of the V-mask (H) at the point of origination by H = h * s. You can select h using an ARL table. See [16] and [17].

k: Type a value greater than 0 for k. For one-sided CUSUMs, k is the allowable "slack" in the process. For two-sided CUSUMs, k is the slope of the V-mask arms. You can select k using an ARL table. See [16] and [17].

When subgroup sizes are unequal, calculate control limits/V-mask

Using actual sizes of the subgroups: Choose to use the subgroups defined in the main dialog box to estimate s.

Assuming all subgroups have size: Choose to estimate the control limits using a specified subgroup size, then type the subgroup size. When subgroup sizes are not equal, each control limit is not a single straight line, but varies with the subgroup size. If the sizes do not vary much, you may want to force the control limits to be constant by entering a sample size in this text box. Only the control limits are affected; the plotted data are not changed.