we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years. At December 31, 2013 , the aggregate deferred revenue balance associated with the SkyMiles Program was $4.4 billion . A hypothetical 1% change in the number of outstanding miles estimated to be redeemed would result in a $28 million impact on our deferred revenue liability at December 31, 2013 .  

 

Goodwill and Indefinite-Lived Intangible Assets  

 

We assess our goodwill and indefinite-lived intangible assets under a qualitative or quantitative approach. Under a qualitative approach, we consider many market factors, including those listed under Key Assumptions below. We analyze these market factors to determine if events and circumstances have affected the fair value of goodwill and indefinite-lived intangible assets. If we determine that it is more likely than not that the asset value may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. Under a quantitative approach, we calculate the fair value of the asset using the Key Assumptions listed below. If the asset's carrying value exceeds its fair value calculated using the quantitative approach, we will record an impairment charge for the difference in fair value and carrying value.  

 

When we evaluate goodwill for impairment using a quantitative approach, we estimate the fair value of the reporting unit by considering market capitalization and other factors. When we perform a quantitative impairment assessment of our indefinite-lived intangible assets, fair value is estimated based on (1) recent market transactions, where available, (2) a combination of limited market transactions and the lease savings method for certain airport slots (which reflects potential lease savings from owning the slots rather than leasing them from another airline at market rates), (3) the royalty method for the Delta tradename (which assumes hypothetical royalties generated from using our tradename) or (4) projected discounted future cash flows (an income approach).  

 

Key Assumptions. The key assumptions in our impairment tests include (1) our projected revenues, expenses and cash flows, (2) an estimated weighted average cost of capital, (3) assumed discount rates depending on the asset and (4) a tax rate. These assumptions are consistent with those hypothetical market participants would use. Since we are required to make estimates and assumptions when evaluating goodwill and indefinite-lived intangible assets for impairment, the actual amounts may differ materially from these estimates. In addition, we consider the amount the intangible assets' fair value exceeded their carrying value in the