proportional free cash flow ( a non-gaap measure ) we define proportional free cash flow as cash flows from operating activities less maintenance capital expenditures ( including non-recoverable environmental capital expenditures ) , adjusted for the estimated impact of noncontrolling interests . the proportionate share of cash flows and related adjustments attributable to noncontrolling interests in our subsidiaries comprise the proportional adjustment factor presented in the reconciliation below . upon the company's adoption of the accounting guidance for service concession arrangements effective january 1 , 2015 , capital expenditures related to service concession assets that would have been classified as investing activities on the consolidated statement of cash flows are now classified as operating activities . see note 1 2014general and summary of significant accounting policies of this form 10-k for further information on the adoption of this guidance . beginning in the quarter ended march 31 , 2015 , the company changed the definition of proportional free cash flow to exclude the cash flows for capital expenditures related to service concession assets that are now classified within net cash provided by operating activities on the consolidated statement of cash flows . the proportional adjustment factor for these capital expenditures is presented in the reconciliation below . we also exclude environmental capital expenditures that are expected to be recovered through regulatory , contractual or other mechanisms . an example of recoverable environmental capital expenditures is ipl's investment in mats-related environmental upgrades that are recovered through a tracker . see item 1 . 2014us sbu 2014ipl 2014environmental matters for details of these investments . the gaap measure most comparable to proportional free cash flow is cash flows from operating activities . we believe that proportional free cash flow better reflects the underlying business performance of the company , as it measures the cash generated by the business , after the funding of maintenance capital expenditures , that may be available for investing or repaying debt or other purposes . factors in this determination include the impact of noncontrolling interests , where aes consolidates the results of a subsidiary that is not wholly-owned by the company . the presentation of free cash flow has material limitations . proportional free cash flow should not be construed as an alternative to cash from operating activities , which is determined in accordance with gaap . proportional free cash flow does not represent our cash flow available for discretionary payments because it excludes certain payments that are required or to which we have committed , such as debt service requirements and dividend payments . our definition of proportional free cash flow may not be comparable to similarly titled measures presented by other companies . calculation of proportional free cash flow ( in millions ) 2015 2014 2013 2015/2014change 2014/2013 change .
what was the percentage change in proportional free cash flow between 2014 and 2015?
( 1 ) service concession asset expenditures excluded from proportional free cash flow non-gaap metric . ( 2 ) the proportional adjustment factor , proportional maintenance capital expenditures ( net of reinsurance proceeds ) and proportional non-recoverable environmental capital expenditures are calculated by multiplying the percentage owned by noncontrolling interests for each entity by its corresponding consolidated cash flow metric and are totaled to the resulting figures . for example , parent company a owns 20% ( 20 % ) of subsidiary company b , a consolidated subsidiary . thus , subsidiary company b has an 80% ( 80 % ) noncontrolling interest . assuming a consolidated net cash flow from operating activities of $ 100 from subsidiary b , the proportional adjustment factor for subsidiary b would equal $ 80 ( or $ 100 x 80% ( 80 % ) ) . the company calculates the proportional adjustment factor for each consolidated business in this manner and then sums these amounts to determine the total proportional adjustment factor used in the reconciliation . the proportional adjustment factor may differ from the proportion of income attributable to noncontrolling interests as a result of ( a ) non-cash items which impact income but not cash and ( b ) aes' ownership interest in the subsidiary where such items occur . ( 3 ) includes proportional adjustment amount for service concession asset expenditures of $ 84 million for the year ended december 31 , 2015 . the company adopted service concession accounting effective january 1 , 2015 . ( 4 ) excludes ipl's proportional recoverable environmental capital expenditures of $ 205 million , $ 163 million and $ 110 million for the years december 31 , 2015 , 2014 and 2013 , respectively. .