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CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Income Statement [Abstract] |  |  |  |
Revenues(1) | [1] | $ 3,828,549 | $ 3,300,223 | $ 2,941,347
Payroll taxes, benefits and workers’ compensation costs |  | 3,146,640 | 2,727,492 | 2,449,737
Gross profit |  | 681,909 | 572,731 | 491,610
Salaries, wages and payroll taxes |  | 301,027 | 259,531 | 229,589
Stock-based compensation |  | 20,425 | 24,345 | 16,643
Commissions |  | 28,957 | 22,773 | 19,288
Advertising |  | 18,554 | 16,686 | 16,447
General and administrative expenses |  | 111,068 | 101,273 | 86,693
Depreciation and amortization |  | 22,842 | 18,182 | 16,644
Total operating expenses |  | 502,873 | 442,790 | 385,304
Operating income |  | 179,036 | 129,941 | 106,306
Other income (expense): |  |  |  |
Interest Income |  | 7,992 | 3,413 | 1,267
Interest Expense |  | (4,668) | (3,213) | (2,396)
Income before income tax expense |  | 182,360 | 130,141 | 105,177
Income tax expense |  | 46,947 | 45,739 | 39,186
Net income |  | 135,413 | 84,402 | 65,991
Less distributed and undistributed earnings allocated to participating securities |  | 1,875 | 1,517 | 1,496
Net income allocated to common shares |  | $ 133,538 | $ 82,885 | $ 64,495
Basic net income per share of common stock (in dollars per share) |  | $ 3.24 | $ 2.02 | $ 1.55
Diluted net income per share of common stock (in dollars per share) |  | $ 3.22 | $ 2.01 | $ 1.54
[1] | (1) Revenues are comprised of gross billings less worksite employee (“WSEE”) payroll costs as follows: Year ended December 31,(in thousands)201820172016 Gross billings$23,830,731$20,173,812$17,932,857Less: WSEE payroll cost20,002,18216,873,58914,991,510Revenues$3,828,549$3,300,223$2,941,347

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