Fair value measurements are market-based measurements, not entity-specific measurements. Therefore, fair value measurements are determined based on the assumptions that market participants would use in pricing the asset or liability. The Company follows a three-level hierarchy to prioritize the inputs used in the valuation techniques to derive fair values. The basis for fair value measurements for each level within the hierarchy is described below:

Warrants issued by the Company in connection with a rights offering originally filed under a Form S-1 registration statement in April 2018 (“Series T Warrants”) and in an underwritten public offering in September 2019 (“Series U Warrants”) are classified as liability instruments initially upon issuance. Because some of the inputs to the Company’s valuation model are either not observable or are not derived principally from or corroborated by observable market data by correlation or other means, the warrant liability is classified as Level 3 in the fair value hierarchy.

The estimated fair value of the Series T Warrants as of September 30, 2020 and December 31, 2019 was determined by using an option pricing model with the following assumptions. The Series T Warrants will be marked to market as of each balance sheet date until they are exercised or upon expiration, with the changes in fair value recorded as non-operating income or loss in the statements of operations and comprehensive loss.

	As of September 30, 2020			As of December 31, 2019
Expected term	0.55 years			1.1 years
Common stock market price	$	2.56		$	2.40
Risk-free interest rate		0.11	%		1.59	%
Expected volatility		128	%		168	%
Resulting fair value (per warrant)	$	1.04		$	1.47

The Company estimated the fair value of the Series U Warrants with the Black Scholes model with the following assumptions. The Series U Warrants that have not been amended (Note 11) will be marked to market as of each balance sheet date until they are exercised or upon expiration, with the changes in fair value recorded as non-operating income or loss in the statements of operations and comprehensive loss.

	As of September 30, 2020			As of December 31, 2019
Expected term	4 years			4.75 years
Common stock market price	$	2.56		$	2.40
Risk-free interest rate		0.22	%		1.68	%
Expected volatility		146	%		135	%
Resulting fair value (per warrant)	$	2.21		$	1.94

The following table summarizes the change in Level 3 warrant liability value (in thousands):

	Three Months Ended						Nine Months Ended
Warrant liability	September 30, 2020			September 30, 2019			September 30, 2020			September 30, 2019
Beginning balance	$	242		$	424		$	6,929		$	916
Issuance		—			10,215			—			10,215
Exercise/settlement		—			(794	)		—			(794	)
Change in fair value		81			561			(2,342	)		69
Reclassification to equity		(240	)		—			(4,504	)		—
Ending balance	$	83		$	10,406		$	83		$	10,406