The IRR, which is consistent with “base case” assumptions, was 15.7%. But, the estimate of IRR was sensitive to variations in different assumptions. In particular, some obvious uncertainties would be the number of units that Boeing would be able to sell and at what price. For example, if Boeing only sold 1,500 units in the first 20 years, then, as shown in Exhibit 9, the IRR would drop to 11%. This might occur if air travel demand worsened, or if Airbus entered this segment with a new competing product.