Trade accounts receivables and unbilled services consist of the following:


(in millions)	September 30, 2024		December 31, 2023
Trade accounts receivable	$	1,176	$	1,473
Unbilled services	2,061		1,942
Trade accounts receivable and unbilled services	3,237		3,415
Allowance for doubtful accounts	(41)		(34)
Trade accounts receivable and unbilled services, net	$	3,196	$	3,381

Unbilled services and unearned income were as follows:


(in millions)	September 30, 2024		December 31, 2023		Change
Unbilled services	$	2,061	$	1,942	$	119
Unearned income	(1,824)		(1,799)		(25)
Net balance	$	237	$	143	$	94

Unbilled services, which is comprised of approximately 67% and 68% of unbilled receivables and 33% and 32% of contract assets as of September 30, 2024 and December 31, 2023, respectively, increased by $119 million as compared to December 31, 2023. 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 $25 million over the same period resulting in an increase of $94 million in the net balance of unbilled services and unearned income between September 30, 2024 and December 31, 2023. 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 nine months ended September 30, 2024, through its accounts receivable factoring arrangements that the Company utilizes most frequently, the Company factored approximately $531 million of customer invoices on a non-recourse basis and received approximately $520 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.