> Are you not sure if you can use the existing implementation? Well, then your project might take 45 minutes, or it might take two weeks. Suddenly you have two

> Are you not sure if you can use the existing implementation? Well, then your project might take 45 minutes, or it might take two weeks. Suddenly you have two orders of magnitude of uncertainty in your estimate. Maybe you know you can't just use the existing implementation; how much of it can you reuse? This. One of the biggest sources of uncertainty is not knowing how much of the existing code will work well enough for you.