U.S. GAAP establishes a three-tier fair value hierarchy to classify and disclose all assets and liabilities measured at fair value on a recurring basis, as well as assets and liabilities measured at fair value on a non-recurring basis, in periods subsequent to their initial measurement. The hierarchy requires us to use observable inputs when available and to minimize the use of unobservable inputs when determining fair value. The three tiers are defined as follows:

•Level 1 - Observable inputs based on unadjusted quoted prices in active markets for identical assets or liabilities;

•Level 2 - Inputs, other than quoted prices in active markets, that are observable either directly or indirectly; and

•Level 3 - Unobservable inputs for which there is little or no market data and which require us to develop our own estimates and assumptions reflecting those that a market participant would use.

The asset or liability’s fair value measurement level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Valuation techniques maximize the use of relevant observable inputs and minimize the use of unobservable inputs. There were no instruments measured at fair value on a recurring basis using significant unobservable inputs as of September 30, 2023 and December 31, 2022.

•Market approach - Uses prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities;

•Income approach - Uses valuation techniques to convert future amounts to a single present amount based on current market expectations about those future amounts; and

•Cost approach - Based on the amount that currently would be required to replace the service capacity of an asset (i.e., replacement cost).

The carrying amounts of our cash, cash equivalents, restricted cash, accounts receivable, accounts payable, and accrued expenses approximate fair value as of September 30, 2023 and December 31, 2022 because of the relatively short duration of these instruments. Additionally, the carrying value of our debt associated with the term loan facility approximates fair value because the interest rates are variable and reset on relatively short durations to the then market rates.

Our investment portfolio consists largely of debt investments classified as available-for-sale. Changes in the fair value of available-for-sale securities, excluding other-than-temporary impairments, are recorded in Other comprehensive income (loss). The components of our investments as of September 30, 2023 are as follows (in thousands):


	As of September 30, 2023
	Fair Value Measurement							Balance Sheet Classification
	Fair Value Level	Cost Basis		Unrealized Gains (Losses)		Market Value		Cash and Cash Equivalents		Short-term Investments and Marketable Securities
Money market fund	Level 1	$	21,034	$	—	$	21,034	$	21,034	$	—
U.S. Treasury bonds	Level 1	17,002		(10)		16,992		—		16,992
Commercial paper	Level 2	12,042		—		12,042		—		12,042

Agency bonds	Level 2	9,694		(2)		9,692		—		9,692
Total investments		$	59,772	$	(12)	$	59,760	$	21,034	$	38,726


	As of December 31, 2022
	Fair Value Measurement							Balance Sheet Classification
	Fair Value Level	Cost Basis		Unrealized Gains (Losses)		Market Value		Cash and Cash Equivalents		Short-term Investments and Marketable Securities
Money market fund	Level 1	$	39,469	$	—	$	39,469	$	39,469	$	—
U.S. Treasury bonds	Level 1	9,396		(13)		9,383		—		9,383
Commercial paper	Level 2	26,704		—		26,704		—		26,704
Corporate bonds	Level 2	9,353		(12)		9,341		—		9,341
Agency bonds	Level 2	2,432		3		2,435		—		2,435
Total investments		$	87,354	$	(22)	$	87,332	$	39,469	$	47,863

There were no Level 3 assets held at any point during the three and nine months ended September 30, 2023 and 2022. Additionally, there were no transfers between Levels 1 and 2 during the nine months ended September 30, 2023 and 2022. Interest income on our investments totaled $2.4 million and $7.1 million for the three and nine months ended September 30, 2023, respectively. Interest income on our investments totaled $0.2 million and $0.5 million for the three and nine months ended September 30, 2022, respectively.

The amortized cost basis and fair value of debt securities as of September 30, 2023, by contractual maturity, are as follows (in thousands):


	As of September 30, 2023
	Cost Basis		Fair Value
Due in one year or less	$	59,772	$	59,760

Total investments	$	59,772	$	59,760

Actual maturities may differ from the contractual maturities in the table above because borrowers have the right to call or prepay certain obligations.