The following process can not be assessed for capability. There are Nelson Tests, Western Electric Tests and often times you can adjust them to create your own conditions to test for process control. Some conditions of special cause can occur within the LCL and UCL (control limits). Recall, that not all out of control conditions are this obvious. Each of the 10 data points has 5 observations within it. Statistical software will normally have the ability to test for conditions that indicate process control or the lack thereof.Įach data point is the mean of a subgroup of 5 observations. The Xbar chart below shows an out of control process. Each data point has >1 observation record and this helps to identify true anomalies or capture a longer term representation of each data point. Xbar-R charts are recommended over I-MR charts primarily due to containing more data (which strengthens a decision). The R chart plots the range of the subgroups and is applied to assess whether the variation from subgroup to subgroup is in control. The I-MR chart obviously only has one observation point for each "group", or data point, so the plot is each point itself. The Xbar chart plots the mean of the each subgroup. These charts are used to verify process control before assessing capability such as Cpk, Ppk, Cp, Pp, or Cpm. There are a few commonly used charts to assess process control If the R chart is out of control, then the control limits on the X-bar chart may be inaccurate and exhibit Type I or II error. The R chart is used to review the process variation which must be in control to correctly interpret the Xbar chart. The control limits of the Xbar chart are calculated with the inputs of the process spread and mean. This control chart, along with I-MR and X-bar
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