Basic and diluted net loss per common share is presented in conformity with the two-class method required for participating securities. Holders of Series F preferred stock were entitled to receive cumulative dividends at the annual rate of 10% compounded quarterly payable prior and in preference to any dividends on any shares of our common stock, subject to certain adjustments as set forth in our certificate of incorporation. In the event a dividend is paid on common stock, the holders of preferred stock were entitled to a proportionate share of any such dividend as if they were holders of common stock (on an as-if converted basis). Accordingly, all of our outstanding series of preferred stock were considered to be participating securities. The holders of our preferred stock did not have a contractual obligation to share in our losses; therefore, no amount of total undistributed loss was allocated to preferred stock. Net loss attributable to common stockholders is calculated as net loss less current period preferred stock dividends.

Basic net loss per share attributable to common stockholders is computed by dividing net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Because we have reported a net loss for both the three and nine-months ended September 30, 2020, and 2019, the number of shares used to calculate diluted net loss per share of common stock attributable to common stockholders is the same as the number of shares used to calculate basic net loss per share of common stock attributable to common stockholders for the period presented because the potentially dilutive shares would have been antidilutive if included in the calculation.

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted-average shares outstanding because such securities have an antidilutive impact due to losses reported:

	Three months ended September 30,				Nine months ended September 30,
(Unaudited, in thousands)	2020		2019		2020		2019
Preferred stock as-converted		—		34,442		—		34,442
Stock options outstanding		9,391		9,422		9,391		9,422
Restricted stock units		1,363		—		1,363		—
Warrants to purchase common stock		—		364		—		364
Convertible debt		—		2,180		—		2,180
Total potentially dilutive securities		10,754		46,408		10,754		46,408