Comparison between Cp/Cpk (Process Capability) and Pp/Ppk (Process Performance)

Tacettin İKİZ · December 14, 2024, 10:39:32 AM

Comparison between Cp/Cpk (Process Capability) and Pp/Ppk (Process Performance)

Cp/Cpk (Process Capability) vs Pp/Ppk (Process Performance)

General Overview

1. Process Capability (Cp/Cpk):
- Measures how well a process can produce outputs within specified limits under ideal conditions (short-term variation).
- Focuses on the inherent potential of the process when it is stable and in control.

2. Process Performance (Pp/Ppk):
- Evaluates how the process actually performs over time, considering both short-term and long-term variations.
- Reflects the real-world ability of the process, even if it is not fully stable.

Detailed Comparison

Aspect	Cp/Cpk (Process Capability)	Pp/Ppk (Process Performance)
Purpose	Evaluates the process's potential capability (short-term, within-group variation).	Assesses the actual process performance over time (long-term variation included).
Data Used	Subgroup-wise or sampling data collected from the process.	Entire population data from the process.
Variation Captured	Short-term, within-subgroup variation (inherent process variation).	Overall variation, including within- and between-subgroup variations.
Sigma Calculation	Uses the range method from subgroups: Code Select Expand `σwithin = R̄ / d₂`	Uses the Root Mean Square (RMS) method: Code Select Expand `σoverall = sqrt(Σ(xi − x̄)² / (n−1))`
Formula	Code Select Expand `Cp = (USL − LSL) / (6σwithin)` Code Select Expand `Cpk = min((USL − μ) / 3σwithin, (μ − LSL) / 3σwithin)`	Code Select Expand `Pp = (USL − LSL) / (6σoverall)` Code Select Expand `Ppk = min((USL − μ) / 3σoverall, (μ − LSL) / 3σoverall)`
Causes of Variation	Accounts only for common causes of variation (process assumed stable).	Includes both common and special causes of variation (process may be unstable).
Applications	Used for process improvement when the process is in control.	Used during PPAP, audits, or for validating unstable processes.
Stability Assumption	Assumes the process is stable and in control.	Does not assume stability; works with shifting or drifting processes.
When to Use	Ideal for improving stable processes.	Suitable for both initial validation and long-term tracking of unstable processes.

Key Points

1. Cp/Cpk:
- Focuses on the potential capability of a process under ideal, stable conditions.
- Ideal for evaluating short-term variation and used in Statistical Process Control (SPC).
- Does not include special causes of variation.

2. Pp/Ppk:
- Evaluates real-world performance, considering all types of variations (common and special causes).
- Suitable for PPAP (Production Part Approval Process) and long-term audits.
- Can be used even if the process is not fully stable.

Conclusion

- Use Cp/Cpk to determine the process's capability to meet specifications under ideal conditions.
- Use Pp/Ppk to assess how the process performs in real-life, considering both short- and long-term variations.

News:

Comparison between Cp/Cpk (Process Capability) and Pp/Ppk (Process Performance)

Tacettin İKİZ