6. NET LOSS PER SHARE ATTRIBUTABLE TO COMMON STOCK

Net loss per share attributable to common stock is calculated using the two-class method. The two-class method is an allocation method of calculating loss per share when a company’s capital structure includes participating securities that have rights to undistributed earnings. Historically, the Company’s employees and officers that held unvested restricted stock were entitled to dividends when the Company paid dividends (“participating”). The Company’s employees and officers that hold unvested restricted stock awarded during 2016 or thereafter are not entitled to dividends when the Company pays dividends (“non-participating”). The Company’s basic net loss per share attributable to common stock is computed by reducing the Company’s net loss by the income allocable to unvested restricted stockholders that have a right to participate in earnings. The Company’s employees and officers that hold unvested restricted stock do not participate in losses because they are not contractually obligated to do so. The undistributed earnings are allocated based on the relative percentage of the weighted average unvested participating restricted stock awards. The basic net loss per share attributable to common stock is computed by dividing the net loss attributable to common stock by the weighted average shares outstanding. The Company’s diluted loss per share attributable to common stock is computed by adjusting basic loss per share attributable to common stock by income allocable to unvested participating restricted stock, if any, divided by weighted average diluted shares outstanding.

A reconciliation of the net loss per share attributable to common stock is as follows (in thousands, except share and per share data):


	Three Months Ended March 31,
	2017		2016
Net loss	$	(9,154)	$	(8,600)
Income allocable to unvested participating restricted stock		—		—
Basic loss attributable to common stock	$	(9,154)	$	(8,600)
Reallocation of participating earnings		—		—
Diluted loss attributable to common stock	$	(9,154)	$	(8,600)
Weighted average common shares outstanding:
Basic		21,659,539		21,629,817
Dilutive common stock options, restricted stock unit awards and non-participating restricted stock awards		—		—
Diluted		21,659,539		21,629,817
Basic loss attributable to a share of common stock	$	(0.42)	$	(0.40)
Diluted loss attributable to a share of common stock	$	(0.42)	$	(0.40)

The Company had a net loss in each of the three months ended March 31, 2017 and 2016. As a result, all stock options, restricted stock unit awards, and non-participating restricted stock awards were anti-dilutive and excluded from weighted average shares used in determining the diluted loss attributable to a share of common stock for the respective periods. The following weighted average numbers of stock options, restricted stock unit awards, and non-participating restricted stock awards have been excluded from the calculation of diluted loss per share attributable to common stock, as their effect would be anti-dilutive for each of the three months ended March 31, 2017 and 2016:


	Three Months Ended March 31,
	2017		2016
Stock options		363,537		423,627
Restricted stock unit awards		253,315		226,813
Non-participating restricted stock awards		79,805		43,978
Total		696,657		694,418

There were no shares of participating restricted stock awards at March 31, 2017 and 2016 included in common stock outstanding.