Sensitivity analysis is a technique in which:

all key input variables are changed to their most likely values to compute a project's net present value (NPV).

all of the input variables are set at their most reasonably expected values to computes a project's internal rate of return (IRR).

the values of key input variables are changed to observe the resulting changes in a project's net present value (NPV) and its internal rate of return (IRR).

a "bad" set and "good" set of financial circumstances are compared with a most likely, or base case, situation.