I have been in several positions where I report metrics. These metrics are usually gathered month over month, represented in numbers or percentages. Occasionally, the Director/VP I reported to would look at the metrics and question me about one or the other. For example: They would ask why is our scrap higher this month than last?
My answers were somewhat complicated. I would first explain to the Director or VP that the process may be well in control, and we may be tampering by making changes without getting to root cause (This would be VERY Bad). I must put the metrics (measured month over month, in time-series order) into a control chart and show them that over the last 6 months, the process is in control and is displaying the same amount of variation over time. What the Director/VP really wants to see is the variation reduced. This is noted by Cp in Capability Analysis.
To do this, we first get the process under control if it isn’t already. This is half the battle. But remember, just because the process is in control, doesn’t mean that the amount of variation is acceptable. The next thing that we must ask is: “Is the process capable of meeting the customer’s (internal or external) specifications. This is where we get into Capability Analysis. This shows whether the process is capable of meeting the customer specifications.
At this point, we run Capability Analysis on the metric. It will show us whether first we have a larger than acceptable variation or not. But believe me, if the Director/VP is complaining, that answer is simply “No.”. This is represented as Cp (Please request information on our SPC and Capability Lean Six Sigma Yellow Belt course). Ultimately you want your Cp (an index) to be at 1.33 or higher. Confidence Intervals are another useful tool at explaining that for this process, we can have 95% confidence that the Cp is going to be between “X” and “Y”. Any reduction/improvement would require a Lean Six Sigma project.
The other part is Cpk. This represents where the mean is centered between the customer specifications and is also represented as an index. Ultimately you want it to be at 1.33 or higher. But usually, the focus is on variation reduction (Cp>= 1.33).
In fact, if you have a Cp higher than 2.0, you can reduce the specification limits and charge more for your product!
A lot of process issues can be fixed by focusing on SPC & Capability. A lot of money and pain can be saved by having a process that’s in control and capable of meeting the customer’s spec. Your employees should be experts at SPC and Capability. They should be able to recognize when there is excessive variation or whether the process is in control at a glance and not just that but have an “Out of Control Action Plan” (OCAP) in place to solve the issue found in SPC & Capability. We can help with this. Reach out to us and we will get back to you ASAP.