Detailed component models may be required for critical elements of the system. This may be appropriate for either investigating part of the complete system, or for validating key assumptions made in a simpler model. Note that there is a parallel activity for building and refining the simple, whole-system model. Modeling and sizing activity is generally iterative. The estimates and models will almost certainly need revising as more detail and/or changed input information becomes available as the development of a system continues. Further refinement of details and changes to requirements, as well as the need to test different solution scenarios, may require changes to the various models. This in turn will require further iterations to adjust, exercise and calibrate the models.