the goldman sachs group , inc . and subsidiaries notes to consolidated financial statements 2030 purchased interests represent senior and subordinated interests , purchased in connection with secondary market-making activities , in securitization entities in which the firm also holds retained interests . 2030 substantially all of the total outstanding principal amount and total retained interests relate to securitizations during 2014 and thereafter as of december 2018 , and relate to securitizations during 2012 and thereafter as of december 2017 . 2030 the fair value of retained interests was $ 3.28 billion as of december 2018 and $ 2.13 billion as of december 2017 . in addition to the interests in the table above , the firm had other continuing involvement in the form of derivative transactions and commitments with certain nonconsolidated vies . the carrying value of these derivatives and commitments was a net asset of $ 75 million as of december 2018 and $ 86 million as of december 2017 , and the notional amount of these derivatives and commitments was $ 1.09 billion as of december 2018 and $ 1.26 billion as of december 2017 . the notional amounts of these derivatives and commitments are included in maximum exposure to loss in the nonconsolidated vie table in note 12 . the table below presents information about the weighted average key economic assumptions used in measuring the fair value of mortgage-backed retained interests. .
what was the change in the weighted average life ( years ) as of december 2018 and december 2017?\\n
in the table above : 2030 amounts do not reflect the benefit of other financial instruments that are held to mitigate risks inherent in these retained interests . 2030 changes in fair value based on an adverse variation in assumptions generally cannot be extrapolated because the relationship of the change in assumptions to the change in fair value is not usually linear . 2030 the impact of a change in a particular assumption is calculated independently of changes in any other assumption . in practice , simultaneous changes in assumptions might magnify or counteract the sensitivities disclosed above . 2030 the constant prepayment rate is included only for positions for which it is a key assumption in the determination of fair value . 2030 the discount rate for retained interests that relate to u.s . government agency-issued collateralized mortgage obligations does not include any credit loss . expected credit loss assumptions are reflected in the discount rate for the remainder of retained interests . the firm has other retained interests not reflected in the table above with a fair value of $ 133 million and a weighted average life of 4.2 years as of december 2018 , and a fair value of $ 56 million and a weighted average life of 4.5 years as of december 2017 . due to the nature and fair value of certain of these retained interests , the weighted average assumptions for constant prepayment and discount rates and the related sensitivity to adverse changes are not meaningful as of both december 2018 and december 2017 . the firm 2019s maximum exposure to adverse changes in the value of these interests is the carrying value of $ 133 million as of december 2018 and $ 56 million as of december 2017 . note 12 . variable interest entities a variable interest in a vie is an investment ( e.g. , debt or equity ) or other interest ( e.g. , derivatives or loans and lending commitments ) that will absorb portions of the vie 2019s expected losses and/or receive portions of the vie 2019s expected residual returns . the firm 2019s variable interests in vies include senior and subordinated debt ; loans and lending commitments ; limited and general partnership interests ; preferred and common equity ; derivatives that may include foreign currency , equity and/or credit risk ; guarantees ; and certain of the fees the firm receives from investment funds . certain interest rate , foreign currency and credit derivatives the firm enters into with vies are not variable interests because they create , rather than absorb , risk . vies generally finance the purchase of assets by issuing debt and equity securities that are either collateralized by or indexed to the assets held by the vie . the debt and equity securities issued by a vie may include tranches of varying levels of subordination . the firm 2019s involvement with vies includes securitization of financial assets , as described in note 11 , and investments in and loans to other types of vies , as described below . see note 11 for further information about securitization activities , including the definition of beneficial interests . see note 3 for the firm 2019s consolidation policies , including the definition of a vie . goldman sachs 2018 form 10-k 149 .