Basic net loss per share is calculated by dividing net loss for the period by the weighted average common shares outstanding during the period. Diluted net loss per share is calculated by dividing the net loss for the period by the weighted average common shares outstanding, adjusted for all dilutive potential common shares, which includes shares issuable upon the conversion of the Convertible Notes, the exercise of outstanding common stock options, the release of restricted stock, and purchases of shares under the ESPP to the extent these shares are dilutive. For 2013, 2012 and 2011, the diluted net loss per share calculation was the same as the basic net loss per share calculation as all potential common shares were anti-dilutive.

Diluted net loss per share does not include the effect of the following potential weighted average common shares because to do so would be anti-dilutive for the periods presented (in thousands):