Trade accounts receivables and unbilled services consist of the following:


(in millions)	June 30, 2023		December 31, 2022
Trade accounts receivable	$	1,339	$	1,329
Unbilled services	1,828		1,624
Trade accounts receivable and unbilled services	3,167		2,953
Allowance for doubtful accounts	(28)		(36)
Trade accounts receivable and unbilled services, net	$	3,139	$	2,917

Unbilled services and unearned income were as follows:


(in millions)	June 30, 2023		December 31, 2022		Change
Unbilled services	$	1,828	$	1,624	$	204
Unearned income	(1,844)		(1,797)		(47)
Net balance	$	(16)	$	(173)	$	157

Unbilled services, which is comprised of approximately 66% and 61% of unbilled receivables and 34% and 39% of contract assets as of June 30, 2023 and December 31, 2022, respectively, increased by $204 million as compared to December 31, 2022. Contract assets are unbilled services for which invoicing is based on the timing of certain milestones related to service contracts for clinical research whereas unbilled receivables are billable upon the passage of time. Unearned income increased by $47 million over the same period resulting in an increase of $157 million in the net balance of unbilled services and unearned income between June 30, 2023 and December 31, 2022. The change in the net balance is driven by the difference in timing of revenue recognition in accordance with Accounting Standards Codification ("ASC") 606, Revenue from Contracts with Customers, primarily related to the Company’s Research & Development Solutions contracts (which is based on the percentage of costs incurred) versus the timing of invoicing, which is based on certain milestones.

The Company has accounts receivable factoring agreements to sell certain eligible unsecured trade accounts receivable, either based on automatic arrangements or at its option, without recourse, to unrelated third-party financial institutions for cash. During the six months ended June 30, 2023, through its accounts receivable factoring arrangements that the Company utilizes most frequently, the Company factored approximately $394 million of customer invoices on a non-recourse basis and received approximately $385 million in cash proceeds from the sales. The fees associated with these transactions were immaterial. The Company has other accounts receivable arrangements for which the activity associated with them is immaterial.