# Control Chart Limits Identify Performance Problems Easily

## You could try to calculate control limits yourself, but ...

QI Macros will do them for you.

**In a stable process:**

68.3% of the data points should fall between ± 1 sigma.

95.5% of the data points should fall between ± 2 sigma.

99.7% of the data points should fall between the UCL and LCL.

**This enables the detection of problems using Control Chart Rules automated by QI Macros.**

## Go Deeper: What are Control Chart Limits?

Control limits distinguish control charts from a simple line or run chart. They are like traffic lanes that help you determine if your process is stable and predictable. Control limits are **calculated** from your data. Control limit formulas are complex and differ depending on the type of data you have.

#### Start drawing control charts now. Download a 30-day Trial of QI Macros

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

### What are control limit lines?

**The Center Line**equals either the average or median of your data.**The****1-2-3 Sigma Lines**. These are simply ±1, ±2 and ±3 sigma from the center line. Sigma is estimated from your data. It's not just a simple standard deviation.

**Upper Control Limit**(UCL) = +3 sigma

**Lower Control Limit**(LCL) = -3 sigma

### Why are there so many formulas for sigma?

The formula for sigma depends on the type of data you have:

- Is it continuous (variable) or discrete (attribute)?
- What is the sample size?
- Is the sample size constant or does it vary?

#### Each type of data has its own unique formula for sigma and, therefore, its own type of control chart.

There are seven main types of control charts (c, p, u, np, individual moving range XmR, XbarR and XbarS.) Plus there are many more variations for special circumstances. As you might guess, this can get ugly. Here are some examples of control limit formulas:

**p Chart formula**

**Individual Moving Range Chart formula**

**X bar R Chart formula**

*** "Introduction to Statistical Quality Control,"**** Douglas C. Montgomery ***

### The secret formula for creating control charts? QI Macros SPC Software!

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

Just select your data and QI Macros does all of the calculations and draws the control chart for you.

QI Macros calculations are tested and accurate.

QI Macros built in code is smart enough to:

- select the right control chart (and formulas) for you.
- highlight unstable points and trends in red.
- identify process changes and recalculate your control limits.

### QI Macros Also 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.

#### Here's what you can do with the click of a button:

- Run the Process Change Wizard to Identify where changes occurred
- Identify Process Changes (i.e. stair step control limits) on your chart
- Ghost a Point - leave data point on a chart but remove it from control limit calculations
- Delete a Point - remove a point from the chart and from control limit calculations
- Recalculate UCL/LCL - recalculate control limits after adding new data

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

- Only
**$329 USD**- less with quantity discounts - No annual subscription fees
- Free technical support

### Easy to Use

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

### Proven and Trusted

- More than 100,000 users
- In More than 80 countries
- Five Star CNET Rating - Virus free