why is the npv method used
Because the NPV method uses a reinvestment rate close to its current cost of capital, the reinvestment assumptions of the NPV method are more realistic than those associated with the IRR method. NPV also has an advantage over IRR when a project has non-normal cash flows. Non-normal cash flows exist if there is a large cash outflow during or at the end of the project. The presence of non-normal cash flows will lead to multiple IRRs. Hence, the IRR method cannot be employed in the evaluation process.