Control Charts are more complicated than most other tools asked about on the PMP Exam, and they tend to give students some trouble.
Here are the 3 most frequently-asked questions my students always have about this tool.
Q1. Why are there 2 sets of limits on Control Charts?
In the diagram above you can see 2 sets of limits:
1. Customer Specs: Upper & Lower (in blue highlight).
The Customer Specs are the limits that come from the customer.
In this example, the customer is requiring that all the tests of the Deliverable the Project Team will build for them MUST fall within a range of 3.0 ppm – 5.0 ppm (parts per million).
2. Control Limits: Upper & Lower (in yellow highlight)
These are the much tighter limits the Project Team wants to test under. Notice from the diagram that there’s a much smaller range between the 2 yellow lines, compared to the broader range between the 2 blue lines.
The control limits specify that all tests MUST fall within a range of 3.5 – 4.5 ppm.
Q2. So the key question is, why should the team test with tighter limits (control limits) than the Customer asked for (customer specs)?
It’s always a risk to quality that conditions in the lab will be different than conditions out in the real world.
For instance, you might test power-brakes on cars inside a lab, and maybe you’ll simulate snow and rain. But out in the real world, there might also be freezing rain, wet snow, hail-storms, sudden downpours, sharp changes in temperature, etc. that were NOT present in the lab.
So if you test for 3.0 – 5.0 ppm in the lab, like the example above (the one from our class) all your tests might fall in that range. But then once your product is out in the real world, the actual results might range for example from 2.8 (too low) through 5.3 (too high).
To guard against this, most quality teams test under control limits which are much narrower than the customer specs. Using the numbers from our sample chart above, we would shoot for the Control Limits of 3.5 – 4.5 in the lab. Because if your tests fall within this narrower range in the lab, you can have a higher confidence-level that your results will fall within the broader range (3.0 – 5.0) in the outside world.
Teaching a lot of PMP for the US Army, they had an expression for this:
“AIM SMALL, MISS SMALL”
Meaning that in marksmanship, it’s best to aim for the very center of the bulls-eye. Because by aiming for the center, you are more likely to hit the target (even if you don’t hit the center, you are less likely to miss the target altogether).
Q3. Some students ask: “But isn’t that Gold-Plating?”
The Answer is “NO” because:
- Gold-Plating means building extra features that the Customer didn’t ask for.
- By aiming for the narrower range, your results will be much less likely to fall outside the Customer Specifications. And will tend to be closer to the bulls-eye.
- If you’re the Customer, is that Gold-Plating? Or doesn’t that sound like high quality?
For more help with the PMP Exam & PM topics:
1. My PMP Exam Prep class for NYU SPS
Next class starts Sunday, September 22
2. My Risk & Change Management class for NYU SPS
Next class starts Tuesday, November 19
3. My book: The Project Management Answer Book, 2nd Edition
4. My book: “Learn The PMP Keywords In Logical Pairs”
Only $3.99 on Amazon Kindle
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