No matter if you call yourself a “lean practitioner” or “six sigma practitioner” or some combination of the two… one “tool” you should have a deep understanding of is the control chart.
I’ve written about control charts before so if you’re not familiar with what they are I’d suggest you check these articles out before pressing on with this article.
- Control Charts Part 1: Learn what control charts are, their history, and why control limits are +/- 3 standard deviations from the mean.
- Control Charts Part 2: Learn about Attribute Data Control Charts (P chart, C chart, and U chart)
- Control Charts Part 3: Learn about Variables Data Control Charts (I MR chart)
With this said, what I’ve discovered is that there are a few “details” related to control charts that many lean and six sigma practitioners aren’t aware of.
As such, I’d like to discuss 5 of these details in this article.
1. Collect 100 Data Points Before Calculating Control Limits
So, in order to ensure your control limits – which are calculated at +/- 3 Sample Standard Deviations from the mean – you should collect at least 100 data points.
If you don’t have at least 100 data points you can still calculate control limits but you should consider the results preliminary.
2. Study MR Chart Before I Chart
The reason this is so critical is because both the I and Xbar Control Limits assume the variability in the process is in statistical control.
So, even though the MR and R charts are shown below the I and Xbar charts we need to look at them first.
Furthermore, if we see any special cause variation in the MR and R charts we should seek to understand and counter the root cause of that variation before we take any action on the I and Xbar charts.
3. Start with Special Causes Tests 1, 2, and 7
These tests were originally developed by Walter Shewhart and are sometimes referred to as the Western Electric Rules.
As it turns out, the statisticians at Minitab have done extensive testing and discovered that tests 1, 2, and 7 are the most useful for evaluating the stability of the Individuals and Xbar charts.
- Test 1 checks to see if any 1 point is greater than 3 standard deviations from the mean.
- Test 2 checks to see if there are at least 9 points in a row on the same side of the mean.
- Test 7 checks to see if there are 15 points in a row within 1 standard deviation of the mean.
Of course you’re definitely free to use any of these 8 tests but, as a starting point, we’d encourage you to at least start with these 3.
4. Rational Subgroups Rock
Next, while the I-MR Chart is extremely powerful it is limited since each data point is created from a sample size of 1.
So, if we’re able to collect more than one sample we should since doing this will allow us to minimize any noise, for lack of a better word, within each subgroup while maximizing our ability to spot any signals, or special causes, between subgroups.
And this is where Rational Subgroups come in.
Formally defined, a Rational Subgroup is one in which multiple samples are collected so that the chance for variation due to special causes occurring within a subgroup is minimized, while the chance for identifying special cause variation between subgroups is maximized.
5. Choose the Correct “Variation” Chart When Using Rational Subgroups
Finally, if we’re able to collect data in Rational Subgroups the “variation chart” we choose depends on the size of the subgroups we collect.
- Xbar-R Chart: We should use the Xbar-R (Range) Chart when our Rational Subgroups are less than 8 but greater than 1.
- Xbar-S Chart: We should use the Xbar-S (Standard Deviation) Chart when our Rational Subgroups are greater than 8.
Now, since the Xbar-S chart uses the Standard Deviation it is more powerful than the Xbar-R chart which simply uses the Range.
If you’re interested in learning much more about control charts and any other lean or six sigma topic I’d encourage you to check out the Lean and Six Sigma Training we’re continuously developing over at Gemba Academy.