Unmasking Process Capability: An In-depth Look into Process Capability Index (Cp)

Last Updated on 13 September 2023

The Process Capability Index (Cp) is a statistical measure that tells us how well a process can meet its specification limits. It is one of the tools in a Six Sigma Ninja’s armory, helping us stay on track with our ongoing improvement projects.

What is Cp?

The Process Capability Index, or Cp, measures a process’s potential capability, which is defined by the part of the voice of the process. It captures the variability of the process relative to the specification boundaries. A Cp of 1.0 is considered acceptable. However, in excellent Six Sigma organizations, a Cp of 1.5 or more is desirable.

How is Cp Calculated?

The Cp is calculated as the difference between Upper Specification Limit (USL) and Lower Specification Limit (LSL) divided by six times the standard deviation (σ):

Cp = (USL - LSL) / 6σ

Why is Cp Important?

In Six Sigma and Lean, we seek to minimize variations in processes—the less variation, the better. Cp allows us to assess the spread in comparison to the ‘voice’ of the customer, defined by the USL and LSL.

A higher Cp score indicates that the process has a smaller spread in relation to the customer’s tolerance range. It shows us the extent of the improvement needed in the process.

Cp: A Practical Example

Assume we run a bakery, and our goal is to bake loaves weighing 950 grams ± 50 grams. Here, USL = 1000g, LSL = 900g (tolerance range is 100g) and let’s assume the standard deviation σ is 10g.

Calculating Cp:

Cp = (1000 - 900) / (6*10) = 1.67

With a Cp greater than 1.5, our process seems to be in excellent condition.

While Cp tells us about the process capability, it doesn’t consider the process mean’s alignment with the specification limits. For that, we look at another index known as Cpk.