United states securities and exchange commission

(1) Includes unallocated expenses and the elimination of intersegment revenue.

Adjusted Diluted Earnings per Share

We also use adjusted diluted earnings per share as a non-GAAP measure of our core operating performance. Adjusted diluted earnings per share excludes the impact of intangible asset amortization and certain pension settlements, transaction costs, and litigation settlements, along with related income taxes because we do not believe these expenses are representative of our core earnings. This supplemental information related to adjusted diluted earnings per share 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.

The effective tax rates used in the U.S. GAAP financial statements were 14.3% and 15.8% for the twelve months ended December 31, 2016 and 2015 , respectively. Excluded items are generally taxed at the effective tax rate. However, after adjusting the underlying annual tax rate to exclude the impact associated with certain non-cash pension settlements in the second and fourth quarters of 2016 and expenses for legacy litigation in the second quarter of 2015, the adjusted effective tax rates for the full years 2016 and 2015 were 16.8% and 17.9% , respectively.
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Reconciliations of this non-GAAP measure to the reported diluted earnings per share are as follows (in millions, except per share data):

Year Ended December 31, 2016	U.S. GAAP				Adjustments				As Adjusted
Operating income	$	1,906			$	512			$	2,418
Interest income	9				—				9
Interest expense	(282		)		—				(282		)
Other income	36				—				36
Income before income taxes	1,669				512				2,181
Income taxes	239				128				367
Net income	1,430				384				1,814
Less: Net income attributable to noncontrolling interests	34				—				34
Net income attributable to Aon shareholders	$	1,396			$	384			$	1,780
Diluted earnings per share	$	5.16			$	1.43			$	6.59
Weighted average ordinary shares outstanding — diluted	270.3								270.3

Year Ended December 31, 2015	U.S. GAAP				Adjustments				As Adjusted
Operating income	$	1,848			$	490			$	2,338
Interest income	14				—				14
Interest expense	(273		)		—				(273		)
Other income	100				—				100
Income before income taxes	1,689				490				2,179
Income taxes	267				122				389
Net income	1,422				368				1,790
Less: Net income attributable to noncontrolling interests	37				—				37
Net income attributable to Aon shareholders	$	1,385			$	368			$	1,753
Diluted earnings per share	$	4.88			$	1.30			$	6.18
Weighted average ordinary shares outstanding — diluted	283.8								283.8

Year Ended December 31, 2014	U.S. GAAP				Adjustments				As Adjusted
Operating income	$	1,966			$	387			$	2,353
Interest income	10				—				10
Interest expense	(255		)		—				(255		)
Other income	44				—				44
Income before income taxes	1,765				387				2,152
Income taxes	334				73				407
Net income	1,431				314				1,745
Less: Net income attributable to noncontrolling interests	34				—				34
Net income attributable to Aon shareholders	$	1,397			$	314			$	1,711
Diluted earnings per share	$	4.66			$	1.05			$	5.71
Weighted average ordinary shares outstanding — diluted	299.6								299.6