Specify how to estimate the parameters for Xbar-R Chart

Stat > Control Charts > Variables Charts for Subgroups > Xbar-R > Xbar-R Options > Estimate

Specify subgroups to estimate parameters

If some subgroups have erratic data because of special causes that you have already corrected, you can omit these subgroups from the calculations to avoid incorrect parameter estimates.

  1. From the drop-down list, specify whether you want to list the subgroups to omit or the subgroups to include.
    • To list the subgroups to omit, select Omit the following subgroups when estimating parameters (eg, 3 12:15).
    • To list the subgroups to include, select Use the following subgroups when estimating parameters (eg, 3 12:15).
  2. List the subgroups. Use a colon to indicate a range of subgroups. Leave a space between each subgroup or range of subgroups.
    For example, to specify subgroups 4, 7, 11, 12, 13, and 14, enter 4 7 11:14.

Method for estimating standard deviation

Choose the method to estimate the standard deviation that Minitab uses to calculate the control limits.

Subgroup size > 1
  • Rbar: Rbar is the average of the subgroup ranges. The Rbar method is a common estimate of the standard deviation and works best with subgroup sizes from 2 to 8.
  • Pooled standard deviation: The pooled standard deviation is the weighted average of subgroup variances, which gives larger subgroups more influence on the overall estimate. The pooled standard deviation method provides a more precise estimate of the standard deviation when the process is in control.

Use unbiasing constant

Select Use unbiasing constant to eliminate bias that can occur when you estimate certain process parameters. Often, the choice to use unbiasing constants depends on company policy or industry standards.