1. What Are Cpk and Ppk?

Cpk (Process Capability Index) and Ppk (Process Performance Index) are statistical measures used to quantify how well a process meets specification limits. Cpk measures short-term capability while Ppk measures long-term performance.

2. How Do The Calculators Work?

The calculators use the formulas:

Where:

• \( USL \) — Upper Specification Limit
• \( LSL \) — Lower Specification Limit
• \( \mu \) — Process Mean
• \( \sigma \) — Short-term Standard Deviation
• \( \sigma_{long} \) — Long-term Standard Deviation

Explanation: These indices measure how centered the process is within the specification limits and how variable the process is relative to those limits.

3. Importance of Process Capability Indices

Details: Cpk and Ppk are crucial for quality control, process improvement, and ensuring products meet customer requirements. Higher values indicate better process capability.

4. Using The Calculator

Tips: Enter all specification limits, process mean, and standard deviations. Ensure standard deviations are greater than zero for valid calculations.

5. Frequently Asked Questions (FAQ)

Q1: What is the difference between Cpk and Ppk?
A: Cpk uses short-term variation (within subgroup) while Ppk uses long-term variation (overall variation). Cpk represents potential capability, Ppk represents actual performance.

Q2: What are acceptable values for Cpk and Ppk?
A: Generally, values ≥1.33 indicate capable processes. Values ≥1.67 are excellent. Values <1.0 may indicate process improvement needed.

Q3: When should I use Cpk vs Ppk?
A: Use Cpk for stable processes with only common cause variation. Use Ppk for new processes or when special cause variation is present.

Q4: Can Cpk or Ppk be negative?
A: Yes, negative values indicate the process mean is outside the specification limits.

Q5: What if my process has only one specification limit?
A: For one-sided specifications, use the appropriate formula (either USL or LSL calculation only).