Process Capability

Process Capability is a continuous improvement practice that uses statistical techniques to quantify the extent to which a stable process meets end user requirements.

It’s a critical tool for ensuring that the products, services, and solutions you create align with what the end user is looking for—whether that’s a consumer, a vendor, or an employee. Does it help them solve a problem? Are they happy with a product’s quality? Is your solution easy-to-use?

Traditionally associated with Six Sigma, Process Capability can be applied to any business process, regardless of industry, to support data-driven decision-making, based on what real users want.

What Is Process Capability?

Process Capability describes a set of statistical techniques used to predict the complete output of a process by analyzing data from a small, random sample.

Practitioners evaluate a process’s ability to deliver on customer expectations by measuring the inherent variation that exists within that process. From there, you’ll be able to determine what needs to change in order to deliver an end product that aligns with user requirements.

During a quality improvement project, a Process Capability analysis is typically done at the beginning of a study to establish a benchmark and again at the end to gauge the impact of any changes made to the process.

Process Capability Meaning

Process Capability is used to measure the relationship between the voice of the customer and the voice of the process:

Process capability = voice of the customer / voice of the process

Organizations can perform a Process Capability analysis and immediately understand whether an existing process effectively delivers the desired outcome—and to what extent—then use that information to drive improvements. Process Capability can also be used as a predictive tool for estimating whether a process will deliver outputs that meet future requirements.

How to Calculate Process Capability

Calculating Process Capability makes three assumptions—normal distribution, a stable process, and a large sample size.

If you can’t verify that a process meets the criteria, you can use process potential (Cp) and process performance (Cpk) to estimate whether a process is capable of meeting customer specifications in the short term. These indices analyze variations within a single sample allowing practitioners to test processes—despite having limited data. The combination of Cp and Cpk give you a solid base for short-term Process Capability.

However, because Cp and Cpk calculations are based on one random subset of a larger population, they aren’t great for estimating long-term success.

Over time, you’ll gather more data, and statistics inevitably shift. To account for that shift, Process Capability uses another pair of indices—Pp and Ppk—often referred to as process performance ratios—which aim to answer whether a process is capable of meeting user requirements in the long term.

What Is a Process Capability Index?

A process is considered "capable" if all metrics that can be used to measure a product/feature produced by that process fall within the control limits. As a point of reference, control limits fall within three standard deviations of the mean in either direction.

These metrics are known as the five Process Capability Indices.

  1. Percent Out of Specification. In cases where the end user has provided upper and lower specification limits, calculating percent out of specification allows you to determine what percentage of data points fall outside of that predefined range. Anything outside of those limits is considered defective and needs to be addressed.
  2. Parts Per Million (PPM). PPM is used to estimate how many products would be defective if you were to produce a million parts using the exact same process. To calculate PPM, you’ll need to multiply percent out of specification by one million.
  3. Potential Capability. Potential Capability enables you to estimate potential Process Capability in the near term. To measure Potential Capability, you’ll need to calculate the Cp and Cpk statistics.
  4. Overall Capability. Overall capability allows you to estimate long-term performance using the Pp and Ppk statistics. This Process Capability index provides a more accurate picture of whether a process can deliver the desired outcome long term, though it can take months or years to generate sufficient data.
  5. Sigma Level. "Sigma" measures the capability of a process to perform defect-free work.

How to Practice Process Capability

Process Capability enables practitioners to track improvement trends over time and be focused on value creation and outcomes.

It’s an essential improvement tool—particularly where customer satisfaction is concerned. But if you haven’t mastered other statistical improvement tactics like Process Mapping, Statistical Process Control, or Graphs, we recommend nailing those down before diving in.

Watch the first video in the Process Capability series for free, and also try our industry-agnostic approach to teaching Lean and Six Sigma tactics, tools, and best practices.

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