Explain why the "ceteris paribus" assumption is crucial for economic modeling, using the relationship between rainfall and agricultural output in Sierra Leone as an example.

The 50 million spent on the half-completed airport is a sunk cost because it is an expenditure that has already been incurred and cannot be recovered, regardless of whether the project is completed or abandoned.

Sunk costs should be ignored in the decision to complete the project because they are irrelevant to the future costs and benefits. Rational decision-making focuses only on the marginal (additional) costs and benefits of continuing versus abandoning the project.

Since new data shows the project is no longer economically viable, continuing would mean incurring additional costs that outweigh any future benefits. The 50 million is already lost, and finishing the project will not recover it, thus it should not influence the decision to proceed.