1. Some remarks on ACEA COP presentation 7 May 2019
Norbert Ligterink

2. Statistics revisited
Statistics is a branch of mathematics dealing with data collection, organization, analysis, interpretation and presentation.
With the lack of data, discussions tend to focus “generic approaches” and “dogmatic definitions”, larded with specific tuned simulations.
The fundamental problem with CoP is the “ambiguous case”:
No decision can be reached with acceptable test burden, if the standard confidence and acceptable risk are predefined in such a borderline case.
In the JRC approach, this leads again to a large fraction of “forced” decisions at the maximum of 16 tests, in such ambiguous cases.
CO2 seems to be the “unsolvable” ambiguous case in textbook statistics (mainly because of European CO2 targets, and Declared Value versus the limits for pollutants).
In the laws of numbers: “95% confidence” levels requires in principle at least 20 tests, and some additional knowledge. (Otherwise a margin is always needed.)

3. Average values and their estimates
Average values, and estimates thereof, are the standard for WLTP pass and fail decisions, because they link directly to the environmental and climate risks.
The measurement values supply important information. On the contrary, “defective rates” do not use this available information, and they are disconnected from average values. (Only restored artificially by an assumption of a specific probability distribution.)
Defective rates are part of generic quality control methods, while the WLTP is specific. Backtracking to defective rates is backtracking from WLTP to Regulation 83 and the limitations thereof.

4. Addressing specific issues
If possible, the different causes of variations in the results must be identified and addressed separately, because they have different effects on the probability distribution (e.g., random versus systematic, and OEM-specific versus generic):
differences between type-approval prototype versus production model
flexibilities in testing
inherent test uncertainty
effects of the interpolation method
effects of the run-in
inter-factory differences and other quality control issues
additional safe margins, in relation with the CO2 targets

5. A car is not a cookie
Textbook examples and 6-sigma protocols have samples of 20 to 50 items tested. (Appropriate for bolts and cookies, but not for vehicles.)
Procedures should be pragmatic and integrated in normal practice.
COP is part of the relation between Type-Approval Authority and the Manufacturer. Any (repeat) COP procedure should be a simple request in this relation, with clear outcome. The associated test burden should not lead to falling short on actual testing and producing results.
TAAs should declare a given COP procedure does not pose inhibitions, in any form, and they are executed regularly. (Maybe COP results should be reported to ensure a common standard.)