Choosing the Right Control Chart

Jay Arthur
888-468-1537
303-756-9144
KnowWare International, Inc.
DBA LifeStar
2253 S. Oneida
Ste 3D
Denver, CO 80224

We work with companies
that want to fire up their profits using
Lean Six Sigma

Copyright © 2011

• Choosing the Right Control Chart
• New QI Macros Control Chart Wizard

Calls received by our tech support line follow the 80/20 rule. 80% of the calls have to do with one question: Which control chart do I use?

Finally, I had a BFO (brilliant flash of the obvious). The QI Macros already analyze your data to figure out how best to graph your data, why not let them automatically figure out which control to use too? Guess what, it works. The Control Chart Wizard is now available in the most recent version of the QI Macros.

If you are still choosing your control chart the old fashioned way, here are some tips.

Choosing a Control Chart

With the recently added XMedianR chart, there are now nine control charts in the QI Macros. How do you know which one to use?

When I'm working with data in Excel, I follow a simple strategy for selecting the right chart based on the format of the data itself. There are three formats I look for:

1. A single row/column.
2. Two rows/columns with a numerator and a denominator.
3. Two or more rows/columns containing multiple observations from each sample.

They look like this:

Single Row/Column

If you only have a single row/column of data, there's only three charts you can use:

• c chart (attribute or counted data) It's always an integer (e.g., 1,2,3,4,5).
• XmR chart (variable or measured data) It usually has decimal places (e.g., 33.75).
• XmR Trend chart for variable data that increases (e.g., rising costs due to inflation)

So which one should you choose? If you're counting indivisible things like defects, people, cars, or injuries, then choose the c chart. If you're measuring things like time, length, weight, or volume, choose the XmR chart. The data looks like this:

c Chart	XmR chart	XmR Trend
defects, people, cars, injuries	time, money, length, weight, or volume	Increasing or decreasing time, money, length, weight, or volume

Look for these patterns in the data and then select the chart.

Two Rows/Columns

If the data has a numerator and a denominator that varies (e.g., defects/batch, errors/transactions), then you will want to use the:

• p chart (one defect maximum per piece)
• u chart (one or more defects per piece)

How can you tell which one to use? I ask myself: "Can this widget have more than one defect?" If yes, use the u chart, otherwise use the p chart.

p Chart	u Chart
Defective items per batch	Defects per item

Sometimes, as in this example, you can have more defects than samples. This is another clue. Again, look for these patterns in the data and then select the chart.

Two or More Rows/Columns of Variable Data

Service industries don't use these charts very often. They are mainly used in manufacturing. If you have two or more rows or columns of variable data (time, weight, length, width, diameter, or volume) then you can choose one of three charts:

• XbarR (Average and Range, 2-10 rows/columns per sample)
• XMedianR (Median and Range, 2-10 rows/columns per sample)
• XbarS (Average and Standard Deviation, 5-50 rows/columns per sample)

Your data should look like this:

You can run the XbarR, XMedianR or XbarS on this data. Xbar uses the average as the measure of central tendency. The XMedianR uses the median. If you have more than five samples per period, then the XbarS will probably be the most robust chart for your needs. You can also use the XbarS if your data has a varying number of samples per period:

Again, look for these patterns in your data and then select the chart.

The np Chart

There's one chart I've left to last because I rarely find situations where it applies. The np chart is like the p chart except that the sample sizes are constant. The data looks like this:

Again, look for these patterns in your data and then select the chart.

Summary

So, just recognizing patterns in your data can make it easier to pick the right control chart.

If you learn to look for these patterns in your data, it will make it easier to choose the right control chart. And it's so easy to draw these charts with the QI Macros, that you can draw them and throw them away if they aren't quite right. For further help, download our SPC Quick Reference Card at http://www.qimacros.com/sustainaid.pdf or consider our 1 hour SPC Simplified Training video on VHS or DVD. See http://www.qimacros.com/sixsig267.html

You can find all of this data in c:\qimacros\testdata.

To purchase the QI Macros

If you already own the QI Macros, you can purchase an upgrade. You must purchase one upgrade for each license you own.

New Control Chart Wizard

Download
QI Macros
30-Day Trial Now!

Or Buy It Now!