The WTG guideline defines an evaluation function in terms of a hit rate q of the prediction model using the "correctly" calculated result values Pi compared to sufficiently significant comparison values Oi:
where
Permissible values for Wrel, W, and the hit rate q are specified in Table 7. These values must be set on a case-by-case basis, with careful assessment of the reference solution's quality. It is important to note that with this evaluation function, critical individual values may be masked by good data in large datasets. Therefore, the assessment should be applied specifically to sub-areas with predominantly the same sign.
Table 7: Example Evaluation Criteria for Force and Pressure Coefficients
| Considered Value | Maximum Absolute Deviation W | Hit Rate q |
|---|---|---|
| Mean Pressure Coefficient | 10% | >90% |
| RMS Value of Pressure Coefficient | 20% | >90% |
| Extreme Value of Unsteady Pressure Coefficient | 20% | >90% |
An alternative evaluation function is the mean squared error e2:
This evaluation function can achieve values as low as e2=0.01, allows a connection to a model factor, and remains informative even with limited data. However, it is susceptible to distortions from individual extreme values, which the percentage agreement method completely excludes. This sensitivity to outliers can lead to an overemphasis on atypical data points, which must be considered when interpreting the results.
| Considered Value | Maximum Absolute Deviation W | Hit Rate q |
| Mean Pressure Coefficient | 10% | >90% |
| RMS Value of Pressure Coefficient | 20% | >90% |
| Extreme Value of Unsteady Pressure Coefficient | 20% | >90% |