Great comments! Class, sometimes you'll have subcontractors on your projects and they will have to develop their own activity durations. You'll need to plug these durations into your project plan, but how can you be sure the estimates are realistic? As you are depending on their expert judgment, it is difficult for you to estimate their activity duration, but on the other hand if you don't verify the estimate your project can take a wild turn if the estimate was aggressive and not realistic.