Process Capability Overview

Overview

This lecture explains the concept of process capability in statistics, focusing on indices like CP, CPK, PP, and PPK, their formulas, interpretations, and examples.

Understanding Process and Process Capability

• A process is a series of actions that transform inputs into outputs.
• Every process consists of input, processing steps, and output.
• Process capability is the ability of a process to produce similar results consistently under given conditions.
• Assessing process capability requires a stable process (only common cause variation) and normally distributed data.
• A large number of samples are needed for accurate assessment.

Process Capability vs. Process Performance

• CP and CPK are process capability indices (short-term, use samples, predict future performance).
• PP and PPK are process performance indices (long-term, use population data, assess past performance).
• Both types measure how well a process meets customer requirements (CTQ: Critical to Quality).

CP, CPK, PP, and PPK Explained

• CP and PP measure how well process data fits within specification limits (USL, LSL).
• CPK and PPK measure how centered the process is relative to specification limits.
• Specification limits are set by the customer and cannot be changed; control limits reflect the process’s variation.

Interpreting Capability Scenarios (Examples)

• Narrow and centered process: high capability, few defects.
• Off-centered or wide process: increased risk of defects, lower capability.
• Ideally, both CP and CPK (or PP and PPK) should be greater than 1 for a capable process.
• CP of 2 and CPK of 2 indicate a highly capable, Six Sigma process.

Formulas and Example Calculations

• CP = (USL - LSL) / (6 × standard deviation)
• CPK = min[(mean - LSL) / (3 × std dev), (USL - mean) / (3 × std dev)]
• Example 1: If CP > 1 but CPK < 1, process is not capable, often due to shift in mean.
• Example 2: If CP = CPK = 2, process is highly capable.

Key Terms & Definitions

• Process Capability — Ability of a process to consistently meet specification limits.
• Specification Limits (USL, LSL) — Customer-defined upper and lower acceptable limits.
• CP — Measure of the process spread relative to specification limits.
• CPK — Measure of process centering and spread relative to specification limits.
• PP/PPK — Long-term performance analogs to CP/CPK.

Action Items / Next Steps

• Practice calculating CP and CPK using provided formulas.
• Answer the three knowledge check questions:
  1. Which index measures potential capability within specs?
  2. Which index includes both variability and centering?
  3. Which index only measures spread of output?
• Review related examples for better understanding.