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The current level of Capability (Cp and Cpk) for each NUD Function is a common summary measure and leading indicator of maturity towards final goals for a technology, product or production process.
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| | Required vs. Measured (Capability) |
- Cp is used to calculate Capability when the mean of a Function or Part or Material Characteristic is "on target".
- Cpk is used to calculate Capability when the mean has shifted "off-target" due to an assignable cause of variation (noise).
A major goal in CPD&M is to use Robust Design methods to minimize σ and to adjust the mean to equal the desired target. Thus Cp/Cpk Growth shows current levels of robustness (Metric #6) and adjustability (Metric #3)!
The formula for calculating the Capability Growth Index for any function or part/material characteristic is:
CGI is reported as a % value of n functions or characteristics within a system or subsystem that are nearing assigned Cp values of 2.0 or Cpk values of 1.5. Cp and Cpk target values can be set at whatever level the project's leadership deems appropriate. Commonly, goals of Cp=2 and Cpk=1.5 are used.
Below is a hypothetical example of n=5 NUD Functional Responses (FR) within a Subsystem under development. Each value is a fraction of 1.00 towards a required target contribution value of .20 per FR. We will multiply this by 100 to get the % growth towards 100%. The 5 FRs' contributions hopefully add up to provide a 1.00 or 100% CGI thus indicating each FR met its goal of a Cp of 2.
- NUD FR 1: 0.13 contribution to overall Subsystem capability
- NUD FR 2: 0.084 contribution to overall Subsystem capability
- NUD FR 3: 0.072 contribution to overall Subsystem capability
- NUD FR 4: 0.20 contribution to overall Subsystem capability (this FR met its goal!)
- NUD FR 5: 0.16 contribution to overall Subsystem capability
Total: .646 * 100 = 64.6% of the way to the goal of all sub-system Cp's = 2. (still have work to do!)
The CGI is conveying a summary value of growth towards a goal of all Functions reaching a Capability goal of Cp=2. This is a common metric of "Six Sigma" performance; meaning the function will fall out of specification ~ 2 times in a billion opportunities to do so. Over time, for your specific phases of development, you will get a sense of how much growth can be expected at certain Gates in your technology or product development processes. Short-falls in growth stand out and provide clear justification for corrective action. Low Cpk is a well-known leading indicator for future reliability problems (the design is too sensitive to noise!). You will know earlier than ever that a potential problem is materializing because you are struggling to grow the Cp and Cpk numbers to the desired levels. You should expect to see the rate of change in capability growth slowing or speeding up in proportion to the efforts you put into development tasks and their enabling CP methods and tools.
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