Quality and Process Methods > Process Capability
Publication date: 08/13/2020

Process Capability

Measure the Variability of a Process over Time

Process capability analysis, used in process control, measures how well a process is performing compared to given specification limits. A good process is one that is stable and consistently produces product that is well within specification limits. A capability index is a measure that relates process performance, summarized by process centering and variability, to specification limits.

Graphical tools such as a goal plot and box plots give you quick visual ways of identifying which process or product characteristics are within specifications. Individual detail reports display a capability report for each variable in the analysis. The analysis enables you to identify variation relative to the specifications or requirements; this enables you to achieve increasingly higher conformance values.

You can specify subgroups to compare the overall variation of the process to the within subgroup variation. You can compute capability indices for processes that produce measurements that follow various distributions. For data that follow none of the specified distributions, you can compute nonparametric capability indices.

Figure 7.1 Example of the Process Capability Platform 


Overview of the Process Capability Platform

Example of the Process Capability Platform with Normal Variables

Example of the Process Capability Platform with Nonnormal Variables

Launch the Process Capability Platform

Process Selection
Process Subgrouping
Moving Range Options
Historical Information
Distribution Options
Other Specifications

Entering Specification Limits

Spec Limits Window
Limits Data Table
Spec Limits Column Property

The Process Capability Report

Goal Plot
Capability Box Plots
Capability Index Plot

Process Capability Platform Options

Individual Detail Reports
Normalized Box Plots
Process Performance Plot
Summary Reports
Make Goal Plot Summary Table

Additional Examples of the Process Capability Platform

Process Capability for a Stable Process
Process Capability for an Unstable Process
Simulation of Confidence Limits for a Nonnormal Process Ppk

Statistical Details for the Process Capability Platform

Variation Statistics
Notation for Goal Plots and Capability Box Plots
Goal Plot
Capability Box Plots for Processes with Missing Targets
Capability Indices for Normal Distributions
Capability Indices for Nonnormal Distributions: Percentile and Z-Score Methods
Parameterizations for Distributions
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