The carrying values of goodwill were $243.5 million and $162.6 million as of December 31, 2022 and 2021, respectively.

A reconciliation of the changes in the carrying value of goodwill during the years ended December 31, 2022 and 2021 is as follows (dollars in thousands):


	As of December 31,
	2022						2021
	CTU		AIUS		Total		CTU		AIUS		Total
Balance, beginning of year	$	73,728	$	88,851	$	162,579	$	45,938	$	72,374	$	118,312
Business acquisitions		59,405		21,556		80,961		27,790		16,477		44,267
Balance, end of year	$	133,133	$	110,407	$	243,540	$	73,728	$	88,851	$	162,579

In assessing the fair value for CTU and AIUS, we performed a qualitative assessment as of October 1, 2022 to determine if we believe it is more likely than not that our reporting unit’s carrying values exceed their respective fair values. When performing the qualitative assessment, management first considered events and circumstances that may affect the fair value of the reporting unit to determine whether it is necessary to perform the quantitative impairment test. Management focused on the significant inputs, including its projections of revenue growth, operating expense leverage and the discount rate used in the prior year quantitative assessment, and any events or circumstances that could affect the significant inputs. These events and circumstances included, but were not limited to, financial performance, future expectations of financial performance, legal, regulatory, contractual, competitive, economic, political, business or other factors, and industry and market considerations, such as a deteriorating operating environment or increased competition. Management evaluated all events and circumstances, including positive or mitigating factors, that could affect the significant inputs used to determine fair value. Additionally, management evaluated its most recent quantitative assessment completed during 2020 to determine by how much the previous fair value exceeded the carrying value for each indefinite-lived intangible asset.

The determination of estimated fair value of each reporting unit requires significant estimates and assumptions, and as such, these fair value measurements are categorized as Level 3 per ASC Topic 820. These estimates and assumptions primarily include, but are not limited to, the discount rate, terminal growth rates, operating cash flow projections and capital expenditure forecasts. Due to the inherent uncertainty involved in deriving those estimates, actual results could differ from those estimates. We evaluate the merits of each significant assumption used, both individually and in the aggregate, to assess the fair value of each reporting unit for reasonableness.

As of December 31, 2022 and 2021, the net book value of intangible assets other than goodwill are as follows (dollars in thousands):


	December 31, 2022										December 31, 2021
	Cost		Accumulated Amortization			Impairments			Net Book Value		Cost		Accumulated Amortization			Net Book Value
Amortizable intangible assets: (1)
Course curriculum	$	2,790	$	(1,461	)	$	-		$	1,329	$	1,400	$	(856	)	$	544
Customer relationships		38,090		(10,815	)		(111	)		27,164		22,300		(5,588	)		16,712
Developed technology		8,820		(1,022	)		(461	)		7,337		2,790		(278	)		2,512
Trade names		13,060		(2,683	)		(543	)		9,834		6,480		(1,940	)		4,540
Net book value, amortizable intangible assets:	$	62,760	$	(15,981	)	$	(1,115	)	$	45,664	$	32,970	$	(8,662	)	$	24,308

Non-amortizable intangible assets:
Accreditation rights									$	1,000						$	1,000
CTU trade name										6,900							6,900
Non-amortizable intangible assets										7,900							7,900
Intangible assets, net									$	53,564						$	32,208

___________________

(1) See Note 3 "Business Acquisitions" for further details on acquired intangible assets.

Amortizable intangible assets are amortized on a straight-line basis over their remaining estimated useful lives, which range from one to fifteen years. Amortization expense was $7.3 million, $4.5 million and $2.8 million for the years ended December 31, 2022, 2021 and 2020, respectively.

During the year ended December 31, 2022, we recorded $1.1 million of asset impairment charges related to certain of our definite-lived intangible assets within the AIUS segment as a result of a decline in cash flows associated with those intangible assets. The fair value of these definite-lived intangible assets were determined to be zero based upon the projected cash flow estimates for the remaining useful life of the primary asset.

As of December 31, 2022, net intangible assets include certain accreditation rights and trade names that are considered to have indefinite useful lives and, in accordance with FASB ASC Topic 350—Intangibles—Goodwill and Other, are not subject to amortization but rather reviewed for impairment on at least an annual basis by applying a fair-value-based test.

We performed our annual impairment testing of other indefinite-lived intangible asset balances as of October 1, 2022 utilizing the qualitative assessment approach and concluded that no indicators existed that would suggest that it is more likely than not that the assets would be impaired. We monitor the operating results and revenue projections related to our CTU trade name and accreditation rights on a quarterly basis for signs of possible declines in estimated fair value. When performing the qualitative assessment, management considered events and circumstances that may affect the fair value of the intangible assets to determine whether it is necessary to perform the quantitative impairment test. These events and circumstances included, but were not limited to, financial performance, future expectations of financial performance, legal, regulatory, contractual, competitive, economic, political, business,

and industry and market considerations. Management evaluated these events and circumstances, including positive or mitigating factors, that could affect the significant inputs used to determine fair value.