Which model structure is considered the most flexible, yet difficult to calibrate?

The hybrid model is viewed as the most flexible because it combines different modeling techniques, such as linear and non-linear approaches, to capture a wide range of relationships in the data. This flexibility allows it to better accommodate complex patterns that may not be easily represented by simpler models.

However, this flexibility comes with the challenge of calibration. Since hybrids can incorporate various elements and methods, determining the appropriate parameters and ensuring they work together effectively requires a deeper understanding of both the underlying data and the modeling techniques involved. The calibration process can be intricate, needing careful adjustment and validation to ensure that the model accurately reflects the data.

In contrast, linear, simple, and additive models generally have more straightforward structures that are easier to calibrate, albeit at the cost of being less adaptable to complex data relationships. This makes them less flexible compared to hybrid models, which can tackle a broader array of scenarios but require more sophisticated calibration efforts.