Basic net income (loss) per share is calculated by dividing the net income (loss) attributable to Merrimack Pharmaceuticals, Inc. by the weighted-average number of common shares outstanding during the period.

Diluted net income (loss) per share is computed by dividing the net income (loss) attributable to Merrimack Pharmaceuticals, Inc. by the weighted-average number of dilutive common shares outstanding during the period. Dilutive shares outstanding is calculated by adding to the weighted shares outstanding any potential (unissued) shares of common stock from outstanding stock options based on the treasury stock method. Outstanding stock options were not included in the diluted net income (loss) per share calculation because the options were out of the money or to do so would have been antidilutive (i.e., the total proceeds upon exercise would have exceeded the market value of the underlying common shares). In a period when a net loss is reported, all common stock equivalents are excluded from the calculation because they would have an anti-dilutive effect, meaning the loss per share would be reduced. Therefore, in periods where a loss is reported, there is no difference in basic and dilutive loss per share.

Stock options are excluded from the calculation of diluted income (loss) per share because the net income (loss) for the three months ended March 31, 2020 and 2019 causes such securities to be anti-dilutive. Outstanding options excluded from the calculation of diluted income (loss) per share for the three months ended March 31, 2020 and 2019 are shown in the chart below:

Three Months Ended

March 31,

(in thousands)

2020

2019

Outstanding options to purchase common stock

1,877

2,148