16. UNAUDITED SELECTED QUARTERLY RESULTS OF OPERATIONS Summarized below is our unaudited quarterly financial data for the quarters ended December 31, 2013 and 2012 (in thousands,  except per share data): Quarter ended 2013 December 31 September 30 June 30 March 31 Services and product sales revenue $ 276,279 $ 273,163 $ 263,139 $ 260,927 Cost of services and product sales 168,393 167,427 163,503 163,645 Other operating expenses 19,969 21,140 17,767 18,245 Operating income 87,917 84,596 81,869 79,037 Interest expense and loss on exchange of Notes 2,483 2,302 2,263 2,269  Income before income tax expense 85,434 82,294 79,606 76,768 Income tax expense 20,718 20,490 19,664 20,036  Net income 64,716 61,804 59,942 56,732 Net income (loss) attributable to non-controlling interest (8) (91) 266 216 Net income attributable to Core Laboratories N.V. $ 64,724 $ 61,895 $ 59,676 $ 56,516 Per share information: Basic earnings per share $ 1.43 $ 1.36 $ 1.30 $ 1.22 Diluted earnings per share (1) $ 1.42 $ 1.35 $ 1.29 $ 1.22 Weighted average common shares outstanding: Basic 45,212 45,526 45,841 46,201 Diluted 45,517 45,828 46,128 46,493 December 31 September 30 June 30 March 31 Services and product sales revenue $ 254,455 $ 245,428 $ 247,006 $ 234,191  Cost of services and product sales 160,958 155,341 156,380 149,140  Other operating expenses 17,629 14,707 18,500 11,145  Operating income 75,868 75,380 72,126 73,906  Interest expense 2,292 2,160 2,178 2,190 Income before income tax expense 73,576 73,220 69,948 71,716  Income tax expense 18,394 18,671 16,997 17,786  Net income 55,182 54,549 52,951 53,930  Net income attributable to non-controlling interest 381 146 35 (21) Net income attributable to Core Laboratories N.V. $ 54,801 $ 54,403 $ 52,916 $ 53,951 Per share information: Basic earnings per share $ 1.18 $ 1.15 $ 1.11 $ 1.13  Diluted earnings per share (1) $ 1.17 $ 1.14 $ 1.11 $ 1.13 Weighted average common shares outstanding: Basic 46,541 47,232 47,473 47,606 Diluted 46,857 47,528 47,791 47,945 (1)  The sum of the individual quarterly diluted earnings per share amounts may not agree with the year-to-date diluted earnings per share amounts as each  quarterly computation is based on the weighted average number of diluted common shares outstanding during that period. Quarter ended 2012 62