Z-MR Chart estimates the mean for each different part or product separately. Z-MR Chart pools all the data for a common part, and obtains the average of the pooled data. The result is the estimate of μ for that part. The part name data define the groupings for estimating the process means. When you use the Relative to size (combine all observations, use ln) option for estimating σ, the means are also taken on the natural log of the data.
You can also center data using historical values. Using historical means allows you to compare your process with past performance. When you use known means to center the data, the chart reflects whether the process is functioning as it has in the past with respect to location. That is, the chart shows whether each part/product mean is the same as previously established. When the process is functioning the same in respect to location, the values are distributed (equally) around the center line.
You can also use nominal specifications—target values—for each part/product to center the data. Using nominal specifications to center the data allows you to compare your process to desired performance. The nominal specifications are established target values for the dimension of interest for each part/product. When you use nominal specifications to center the data, the chart reflects whether the process is producing parts/products that are on target, or whether the process is biased.