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 uses short-term variation while Ppk uses long-term variation.

2. How Does the Calculator Work?

The calculator uses the following formulas:

Where:

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

Explanation: Both indices measure how centered the process is within the specification limits and how much variation exists in the process.

3. Importance of Cpk and Ppk Calculation

Details: These indices 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, mean, and standard deviations. Values must be valid (standard deviations > 0). The calculator will compute both Cpk and Ppk indices.

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 including between subgroup variation).

Q2: What are acceptable Cpk/Ppk values?
A: Generally, values ≥1.33 indicate capable processes, ≥1.67 indicate good capability, and ≥2.0 indicate excellent capability.

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/Ppk be negative?
A: Yes, negative values indicate the process mean is outside the specification limits.

Q5: How are these calculated in Minitab?
A: Minitab provides both Cpk and Ppk values in capability analysis reports along with graphical representation of the process.