Process Performance (Pp and Ppk) Index
Calculate Process Performance (Pp) and Performance Index (Ppk) instantly. Analyze long-term capability, account for process drift, and ensure your actual production meets customer specifications over time.
Updated December 2025
PP
--
Potential Performance
PPK
--
Actual Performance
∿ Performance Distribution
● Limits ● Long-Term Data
LSL Mean USL
The Amber Curve represents your Actual Performance (Long Term). It often appears wider and flatter than the Cpk curve because it includes all variation sources (shifts, drifts, tool wear) over time.
Understanding Pp & Ppk
A comprehensive guide to Long-Term Process Performance. Learn why "Real Life" data often looks different than the pilot run.
Visualizing "Real" Performance
Cpk assumes your process is stable. Ppk assumes reality happens. Watch how long-term data (Ppk) accumulates shifts, drifts, and tool wear that short-term data (Cpk) ignores.
Click to simulate process behavior...
What to look for:
Short Term: The data stays tight around the center. This is your "Potential".
Long Term: The process drifts up and down (e.g., as the room warms up or tools wear). Ppk includes all this variation, which is why Ppk is usually lower than Cpk.
Interpreting Ppk Results
Ppk is your "Actual Performance" index. It answers the question: "Regardless of how capable we think we are, how did we actually perform over the last month?"
< 1.0 PPK
Poor Performance
The process actually produced defects. The variation over the long run was wider than the limits.
Action: 100% Inspection Required 1.0 - 1.33 PPK
Acceptable (Existing)
For an existing stable process, this is often acceptable. You are fitting inside the limits, but with little room for error.
Action: Monitor Closely 1.67+ PPK
New Process / PPAP
For New Parts, customers demand a higher Ppk (often 1.67) to prove the process is robust enough to handle future shifts.
Action: Approve for Production Performance Meter
Pp vs. Ppk: The Difference
Just like Cp/Cpk, we distinguish between "The Width" and "The Location". However, for Pp/Ppk, we calculate "Sigma" differently—we use all data points.
The Width
Pp (Process Performance)
"Could we fit?"
Pp looks at the overall spread of your data. If you collected data for a month, Pp asks: "Is this pile of data narrow enough to fit inside the limits?"
The Location
Ppk (Performance Index)
"Did we fit?"
Ppk checks if that pile of data was actually centered. If your process drifted close to the edge, your Ppk will be low.
Pp: 2.0
Ppk: 2.0
Perfect! The overall process variation is low (High Pp) and centered (High Ppk).
Ppk cannot exceed Pp
Just like Cpk, your Ppk is limited by your Pp. You cannot perform better than your total variation allows, no matter how much you center the process.
The Formulas
The math looks identical to Cpk, with one critical difference: Sigma. In Ppk, we calculate the standard deviation of the entire dataset together.
Potential
Pp Formula
Pp=
USL - LSL 6s
We divide the tolerance range by the Overall Standard Deviation (s). This represents 6-sigma of all variation.
Performance
Ppk Formula
Ppk=min
USL - μ 3s
μ - LSL 3s
We calculate the distance from the Process Mean to the nearest limit, divided by 3 times the Overall Standard Deviation (s).
Variable Key
s
Overall Std Dev
Calculated using the standard root-mean-square formula on ALL data.
μ
Process Mean
The average of your entire dataset.
LSL
Lower Spec
The minimum value allowed by the customer.
Troubleshooting Low Ppk
Because Ppk includes "Long Term" data, a low score often points to issues that happen over time rather than instantly.
Problem A
Process Drift
Symptom: Low Pp & Low Ppk.
Tool Wear / Thermal Shift
Is the machine changing as it warms up? Does the cutting tool wear down during the shift? Ppk captures this movement.
Material Batches
Did you switch raw material batches halfway through the run? Variation between batches destroys Ppk.
Problem B
Setup Consistency
Symptom: High Cpk but Low Ppk.
Shift-to-Shift Setup
Day shift sets the machine at 10.0mm. Night shift sets it at 10.2mm. Both are "capable" individually (High Cpk), but together they create a wide distribution (Low Ppk).
Operator Differences
Standardize how operators load parts or measure. Human inconsistency adds to the overall variation.
Expert Knowledge
Common Questions
What is the main difference between Cpk and Ppk?
Cpk (Capability) predicts the future based on a stable run (Potential).
Ppk (Performance) reviews the past reality.
Ppk (Performance) reviews the past reality.
Note: Ppk captures the shifting and movement over time.
Why is Ppk usually lower than Cpk?
Because Ppk includes ALL sources of variation. It adds up the "Normal Noise" plus "Tool Wear" plus "Operator Shifts".
When should I use Ppk?
Use Ppk for PPAP (New Product Launch) and Long Term analysis. Customers trust Ppk because it proves robustness.
Verified Expert
Disclaimer: This tool is for informational and educational purposes only. Calculations are based on standard formulas but may not account for unique business variables. We do not accept liability for decisions made based on these results.