If I understand correctly, tests are for what the code should do. There is some business logic and you are asserting that the business logic does what it should

If I understand correctly, tests are for what the code should do. There is some business logic and you are asserting that the business logic does what it should do (the test). If you manipulate the business logic, and it no longer does what the test says the code should do, the test fails. Here it is not as clear what the code should do. What should the amount of excess deaths be? In what ways would we change the logic such that the test case would break? If the input data set is static, isn't it more like a mock anyway? I think for this reason you often see more sanity checks in research code because the should case is not as clearly defined.