What is a Control Chart?

This is the start of a four-part blog on control charts. The blogs will answer the following questions:

• What is a control chart?
• What is variation and how does it relate to a control chart?
• What is a control chart used for?
• Where is a control chart used?

Control charts are at the heart of statistical process control (SPC). So maybe we should start with answering what a process is. A process is simply what we do. It can be filling out an expense report, checking a person into a hospital, driving to work, filling a prescription, etc...

Cpk vs. Ppk: Who Wins? [video]

This week’s blog concludes our series on process capability and asks which is better: Cpk or Ppk?

Your supplier has sent you the process capability chart you requested. Looks like your supplier is really performing for you. You note that his process has a Ppk = 1.14 and a Cpk = 2.07. Why are those different? Well, it doesn’t matter. The Cpk is above 1.33, which is what you asked the supplier for. You just missed a very important piece of information about your supplier’s performance. Know what it is?

The accompanying video explores Cpk and Ppk in more detail.

The...

What is Ppk?

Our first two blogs in this process capability series answered two questions: What is process capability? – and - What is Cpk? This blog answers the next question: What is Ppk?

The answer is quite simple. Just to refresh your memory, Cpk is expressed as the following:

Cpk = Minimum (Cpu, Cpl)

Cpu=(USL- X )/3σ

Cpl=( X -LSL)/3σ

where Cpu is the capability based on the upper specification limit (USL), Cpl is the capability based on the lower specification limit (LSL), X...

What is Cpk?

Our first blog in this process capability series explored the simple question “what is process capability?”. There were two parts to the answer:

Process capability is the range of values that you can expect to get from the process over an extended time period.

A process is capable if the range of expected values fall within the specification limits.

Since you are comparing two ranges – the range of output from the process to the range of the specs - it is natural to ask questions about how much these two ranges are different or overlap. This gave rise to numerical...

What is Process Capability?

You hear the term process capability a lot these days. Customers want to know your process capability. You want to know your supplier’s process capability. So, what is meant by the words process capability? For a process:

Process capability is the range of values that you can expect to get from the process over an extended time period.

Suppose your process involves filling bags with sand. Each bag is supposed to hold 50 pounds of sand. The process capability for this process is simply the range of values you expect for the bag weights over an...

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