(1) Represents revenue not recognized under GAAP due to acquisition accounting adjustments associated with the accounting for deferred revenue in significant business combinations.
(2) Includes a) nonrecurring severance expenses of $0.4 million and nonrecurring acquisition related costs of $0.6 million, for the twelve months ended December 31, 2019, and b) an impairment charge for royalty contracts resulting in $1.0 million of expenses for the twelve ended December 31, 2019.
Includes a) nonrecurring costs from the acquisition of Datawatch of $10.4 million for the three and twelve months ended December 31, 2018, b) a gain on the sale of a building of $4.4 million for the twelve months ended December 31, 2018, b) an impairment charge for royalty contracts and trade names resulting in $0.2 million and $2.8 million for the three and twelve months ended December 31, 2018, respectively and c) a non-recurring adjustment for a change in estimated legal expenses resulting in $2.0 million of income for the twelve months ended December 31, 2018.

The following table provides a reconciliation of Adjusted EBITDA and Modified Adjusted EBITDA to net (loss) income, the most comparable GAAP financial measure:

		(Unaudited)
		Three Months Ended December 31,								Twelve Months Ended December 31,
(in thousands)		2019				2018				2019				2018
Net (loss) income		$	(1,502	)		$	(9,003	)		$	(7,542	)		$	15,535
Income tax expense			3,715				5,872				10,930				11,489
Stock-based compensation expense			2,944				2,126				8,528				3,339
Interest expense			2,785				108				6,371				200
Interest income and other (1)			(893	)			9,986				(260	)			4,883
Depreciation and amortization			5,686				3,839				21,522				14,734
Adjusted EBITDA			12,735				12,928				39,549				50,180
Acquisition related deferred revenue (2)			2,250				—				9,000				—
Modified Adjusted EBITDA		$	14,985			$	12,928			$	48,549			$	50,180

(1) Includes a) nonrecurring severance expenses of $0.4 million and nonrecurring acquisition related costs of $0.6 million, for the twelve months ended December 31, 2019, and b) impairment charges for royalty contracts resulting in $1.0 million of expense for the twelve months ended December 31, 2019.