Control Limits - the Key to Control Charts

Control Limits are Calculated from Your Data

Control limits distinguish control charts from a simple line graph or run chart.
Control limits are calculated from your data. They are often confused with specification limits which are provided by your customer.

Control Limits on a Control Chart

control chart limits

 

Control limit calculations begin with the Center Line. It represents the average or median of the data.

Next calculate sigma. The formula used for sigma depends on the type of data being used.

From the center line, lines are drawn at ± 1 sigma, ± 2 sigma and ± 3 sigma.

+ 3 sigma = Upper Control Limit (UCL)
- 3 sigma = Lower Control Limit (LCL)


Control Chart Formulas and Calculations

There are seven main types of control charts and many more variations. Each uses its own distinct formula.

p chart formula example
Individuals chart formula
xbar r chart formula

You Can Try and Calculate Control Limits Yourself, But ...

  • It will suck up a bunch of your time.
  • You'll probably make mistakes and aggravate customers.
  • And you will find your homegrown template hard to maintain.

Based on calls to our tech support line, most people who try to perform manual calculations or build their own Excel formulas end up with incorrect results.


Instead, Use Proven Software like QI Macros Add-in for Excel

QI Macros is an easy to use add-in for Excel that installs a new tab on Excel's toolbar.

QI Macros calculations are tested and accurate and it has additional features like highlighting unstable points and trends in red.

QI Macros wizard will select the right chart and formulas for you based on your data.

QI Macros control chart

How Control Chart Limits are Calculated in QI Macros

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 data after the process change occurred.

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.

The center line is then used to calculate the 1 and 2 sigma lines and the upper control limit and lower control limit.

To check which points are used to calculate your center line, simply click on the data sheet created by the QI Macros (e.g. pdata1) and then 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.

control chart limit calculations


QI Macros Makes it Easy to Update Control Limit Calculations

Once you create a control chart using QI Macros, you can easily update the control limits using the QI Macros Chart Tools menu. To access the menu, you must be on a chart or on a chart embedded in a worksheet.

update control charts

Change the data points used to calculate control limits:

There are also options to easily re-run stability analysis after changing data or control limit calculations.


Why Choose QI Macros Control Chart Software for Excel?

affordable

Affordable

  • Only $249 USD - less with quantity discounts
  • No annual fees
  • Free technical support

easy to use

Easy to Use

  • Works right in Excel
  • Create a chart in seconds
  • Easy to customize & share charts

proven and trusted

Proven and Trusted

  • 100,000 users in 80 countries
  • Celebrating 20th Anniversary
  • Five Star CNET Rating - Virus free