1. What is Process Capability Index (Cp)?

The Process Capability Index (Cp) is a statistical measure that compares the width of the specification limits (USL - LSL) to the width of the process variation (6σ). It indicates how well a process can meet specifications, assuming the process is centered.

2. How Does the Calculator Work?

The calculator uses the Cp formula:

Where:

• \( USL \) — Upper Specification Limit
• \( LSL \) — Lower Specification Limit
• \( \sigma \) — Standard deviation of the process

Interpretation:

• Cp < 1 — Process not capable (variation exceeds specifications)
• 1 ≤ Cp < 1.33 — Marginally capable
• Cp ≥ 1.33 — Capable process
• Cp ≥ 2 — Six Sigma quality

3. Importance of Cp Calculation

Details: Cp helps manufacturers quantify how well their process can produce parts within specifications. It's fundamental in quality control and Six Sigma methodologies.

4. Using the Calculator

Tips: Enter USL, LSL, and standard deviation in consistent units. Standard deviation must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between Cp and Cpk?
A: Cp assumes the process is centered, while Cpk accounts for process centering. Cpk is generally more conservative.

Q2: How many data points are needed for reliable σ?
A: Typically 25-30 data points minimum for a reliable standard deviation estimate.

Q3: What if my process isn't normally distributed?
A: Cp may not be appropriate. Consider non-normal capability indices or transform the data.

Q4: Can Cp be negative?
A: No, since USL > LSL and σ is always positive, Cp is always ≥ 0.

Q5: How often should Cp be calculated?
A: Regularly as part of process control, especially after process changes.