on may 20 , 2015 , aon plc issued $ 600 million of 4.750% ( 4.750 % ) senior notes due may 2045 . the 4.750% ( 4.750 % ) notes due may 2045 are fully and unconditionally guaranteed by aon corporation . we used the proceeds of the issuance for general corporate purposes . on september 30 , 2015 , $ 600 million of 3.50% ( 3.50 % ) senior notes issued by aon corporation matured and were repaid . on november 13 , 2015 , aon plc issued $ 400 million of 2.80% ( 2.80 % ) senior notes due march 2021 . the 2.80% ( 2.80 % ) notes due march 2021 are fully and unconditionally guaranteed by aon corporation . we used the proceeds of the issuance for general corporate purposes . credit facilities as of december 31 , 2015 , we had two committed credit facilities outstanding : our $ 400 million u.s . credit facility expiring in march 2017 ( the "2017 facility" ) and $ 900 million multi-currency u.s . credit facility expiring in february 2020 ( the "2020 facility" ) . the 2020 facility was entered into on february 2 , 2015 and replaced the previous 20ac650 million european credit facility . each of these facilities is intended to support our commercial paper obligations and our general working capital needs . in addition , each of these facilities includes customary representations , warranties and covenants , including financial covenants that require us to maintain specified ratios of adjusted consolidated ebitda to consolidated interest expense and consolidated debt to adjusted consolidated ebitda , tested quarterly . at december 31 , 2015 , we did not have borrowings under either the 2017 facility or the 2020 facility , and we were in compliance with the financial covenants and all other covenants contained therein during the twelve months ended december 31 , 2015 . effective february 2 , 2016 , the 2020 facility terms were extended for 1 year and will expire in february 2021 our total debt-to-ebitda ratio at december 31 , 2015 and 2014 , is calculated as follows: .

2015: net income: 1422
interest expense: 273
income taxes: 267
depreciation of fixed assets: 229
amortization of intangible assets: 314
total ebitda: 2505
total debt: 5737
total debt-to-ebitda ratio: 2.3
2014: net income: 1431
interest expense: 255
income taxes: 334
depreciation of fixed assets: 242
amortization of intangible assets: 352
total ebitda: 2614
total debt: 5582
total debt-to-ebitda ratio: 2.1

we use ebitda , as defined by our financial covenants , as a non-gaap measure . this supplemental information related to ebitda represents a measure not in accordance with u.s . gaap and should be viewed in addition to , not instead of , our consolidated financial statements and notes thereto . shelf registration statement on september 3 , 2015 , we filed a shelf registration statement with the sec , registering the offer and sale from time to time of an indeterminate amount of , among other securities , debt securities , preference shares , class a ordinary shares and convertible securities . our ability to access the market as a source of liquidity is dependent on investor demand , market conditions and other factors. .

Questions:
what was the interest expense in 2015?
and what was it in 2014?
what was, then, the change over the year?
and how much does this change represent in relation to the 2014 interest expense, in percentage?

Answers:
273
255
18
7.1%