Control Limit Stages or Phases on a Control Chart
Use Stair Step Limits to Show a Process Change on Control Charts
Generally, you calculate control limits using your first 20 to 25 data points and then you use those limits to evaluate the rest of your data. If you have a process change, you should recalculate your control limits beginning with the first point after the process change occurred.
QI Macros makes it easy to show process changes on control charts. The process is different depending on how you create your control chart:
- New Charts Created with a Macro
- Existing Charts Created with a Macro
- Control Chart Templates
- Excel 2007 Workaround for Showing Process Change on Embedded Charts
To calculate two or more sets of control limits on new charts simply leave a blank row between the data points where you want the limits to change:
Select the data including the blank row(s) and run the chart. You should get a chart with two or more sets of control limits:
When you select data and then run a control chart using QI Macros, QI Macros will use ALL of the data points selected to calculate the center line (ie. average). If you have 14 points it will use 14 points, if you have 26 points it will use 26 points, etc. To check which points are used to calculate your center line, simply move the chart to reveal the calculations behind it. Click on the first cell under the cell labeled "Average". You can determine the data range used by viewing the formula in Excel's formula bar.
In the following example, we clicked on cell H2 and noted that the average is calculated using cells B2 to B26 and C2 to C26.
To show a process change on an existing chart.
- Go to the chart and click on the first data point where the process change occurred.
Tip: The first time you click on a point, Excel will select the whole line. Click on the point a second time to select just the point. If you have done this correctly, Excel will highlight your point.
- Click on QI Macros chart menu, select the "Process Changes" drop-down and choose Show Process Change:
- The macros will calculate a new set of control limits starting at the data point you selected. Note: you can do this more than once on a chart. The example below shows three separate sets of control limits:
Don't forget to rerun stability analysis using the new control limits. Click on the chart sheet. Use QI Macros menu to select "Analyze Stability With Control Chart Rules." QI Macros will rerun stability analysis using the new control limits.
- Note: Performing these steps on an X chart will not update the R chart. You will need to update the R chart separately.
Use our Process Change Wizard!
The QI Macros Process Change Wizard will analyze your data and locate potential runs and/or trends.
Want to Create an Interrupted Time Series Using QI Macros? See Instructions Below:
You can create an Interrupted Time Series chart by simply performing a Show Process Change at the end and beginning of your two different series!
To do so, place a blank row in between your two data sets, and create your Control Chart:
Why Choose QI Macros Over Other Control Chart Software?
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