X and S - Options - Limits
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Stat > Control Charts > Variables Charts for Subgroups > Xbar-S > Xbar-S Options > Limits

Allows you to draw control limits above and below the mean at the multiples of any standard deviation. You can set bounds on the upper and/or lower control limits. When the calculated upper control limit is greater than the upper bound, a horizontal line labeled UB is drawn at the upper bound instead. Similarly, if the calculated lower control limit is less than the lower bound, a horizontal line labeled LB is drawn at the lower bound instead. You can also force the control limits to be constant when sample sizes are unequal.

For an example, see Example of an Xbar chart with tests and customized control limits.

Dialog box items

Display additional sigma limits at

These multiples of the standard deviation: Enter one or more values. The values entered are the number of standard deviations above and below the center line. If you specify more than one value, a pair of control limits is drawn for each number in the list.

Place bounds on control limits of Xbar Chart

Lower standard deviation limit bound: Check to set a lower bound for the control limits for the X chart. If the calculated lower control limit is less than the lower bound, a horizontal line labeled LB will be drawn at the lower bound instead.

Upper standard deviation limit bound: Check to set an upper bound for the control limits for the X chart. If the calculated upper control limit is greater than the upper bound, a horizontal line labeled UB will be drawn at the upper bound instead.

Place bounds on control limits of S Chart

Lower standard deviation limit bound: Check to set a lower bound for the control limits for the S chart. If the calculated lower control limit is less than the lower bound, a horizontal line labeled LB will be drawn at the lower bound instead.

Upper standard deviation limit bound: Check to set an upper bound for the control limits for the S chart. If the calculated upper control limit is greater than the upper bound, a horizontal line labeled UB will be drawn at the upper bound instead.

When subgroup sizes are unequal, calculate control limits

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 enter 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.