7. SHARE-BASED COMPENSATION

Share-Based Compensation

We issue share-based compensation to employees in the form of stock options, restricted stock units (RSUs), and performance stock units (PSUs). We account for the share-based awards by recognizing the fair value of share-based compensation expense on a straight-line basis over the service period of the award, net of estimated forfeitures. The fair value of stock options is estimated on the grant date using the Black-Scholes option pricing model. The fair value of RSUs and non-executive PSUs is based on the closing price of our common stock on the grant date. Executive PSUs are valued using a binomial option pricing model using the following inputs: stock price, volatility, and risk-free interest rates. Changes in estimated inputs or using other option valuation methods may result in materially different option values and share-based compensation expense.

The following table summarizes the amount of share-based compensation expense by line item on the statements of operations:

Share-based Compensation Expense by Line Item

Options activity and positions

The following table summarizes shares, weighted-average exercise price, weighted-average remaining contractual term and aggregate intrinsic value of options outstanding and options exercisable as of September 30, 2021:

Option Positions

As of September 30, 2021, our unrecognized share-based employee compensation related to stock options was $103,000 which we plan to expense over the next 0.8 years.

Restricted stock activity and positions

The following table summarizes activity and positions with respect to RSUs and PSUs for the nine months ended September 30, 2021:

Restricted Stock Unit Activity

As of September 30, 2021, our unrecognized share-based compensation related to RSUs was $26.0 million which we plan to expense over the next 2.5 years and our unrecognized share-based compensation related to the non-executive PSUs was $1.2 million, which we plan to expense over the next 1.0 years.