AKA | Capability Indices |
Classification | Analyzing/Trending (AT) |
Process capability ratios are calculated to determine the process variation that comes from natural or special causes. These ratios, also called Cp and Cpk Indices, relate the process variability to the design specification (tolerance) that reflects the customer's expectation or requirements. Cp is listed to characterize the capability, and Cpk is used to measure actual process performance.
To estimate how well the process meets customer requirements.
To monitor and measure product quality.
To verify process variability to design specifications.
To promote communications among design engineering, suppliers, and manufacturing.
→ | Select and define problem or opportunity |
→ | Identify and analyze causes or potential change |
Develop and plan possible solutions or change | |
Implement and evaluate solution or change | |
→ | Measure and report solution or change results |
Recognize and reward team efforts |
Research/statistics | |
Creativity/innovation | |
1 | Engineering |
Project management | |
2 | Manufacturing |
Marketing/sales | |
Administration/documentation | |
Servicing/support | |
3 | Customer/quality metrics |
4 | Change management |
before
Standard Deviation
Sampling Methods
Descriptive Statistics
Normal Probability Distribution
Control Chart
after
Variance Analysis
Process Flowchart
Process Analysis
Activity analysis
Basili Data Collection Method
Definitions of process capability ratios:
Cp: A measure of ideal or potential process capability. The Cp index reflects the best ability of a process to perform within lower and upper design specification limits (LSL ⃡ USL).
Cpk: A measure of actual or located process performance. The Cpk index reflects the actual, located process mean relative to the design target value.
CPU: Upper process capability
CPL: Lower process capability
μ or : Process average
Equations for calculating Cp and Cpk:
Process capability:
STEP 1 A team is formed with the goal of reduced process variability.
STEP 2 Through the data collection process, process data such as specific product specifications tolerances, control charts, samples of measurements, and historical data calculations, are acquired. See example Fax Paper Extension Wire Measurements.
STEP 3 Sample data are organized and summarized using descriptive statistics. A recommendation is made to read some background tools contained in this book:
Descriptive statistics (tool 66)
Normal probability distribution (tool 119)
Standard deviation (tool 184)
STEP 4 Using process means and standard deviations, the Cp capability ratios can be calculated and compared.
Note | Measurement in this example is centimeters (cm). |
STEP 5 An action plan is developed on the basis of the actual process performance results calculated. Required activities usually require reduction of process variations or recentering of the process within the specifications.
Fax Paper Extension Wire Measurements
Cp—Process capability and variability