NOTE 6. STOCK INCENTIVE PLAN

The company has one stock incentive plan. The Amended and Restated 1999 Stock Incentive Plan (the “Plan”) allows the company to grant stock options and restricted stock to employees, directors, officers and advisors of the company. A maximum of 5,000,000 shares are authorized under the Plan. Options generally vest over a four-year period and have a maximum term of five years from the vesting date. The Plan provides that vesting may be accelerated upon the occurrence of certain corporate transactions of the company. The Plan provides that the Board of Directors, or a committee appointed by the Board, has discretion, subject to certain limits, to modify the terms of outstanding options. We recognize non-cash stock-based compensation expense related to the estimated fair value of stock options granted in accordance with FASB ASC Topic 718, Compensation—Stock Compensation.

The following table reflects the components of stock-based compensation expense recognized in the Condensed Consolidated Statements of Operations for the three months ended March 31, 2014 and 2015:

Stock option and restricted stock grants

The Plan allows the company to grant stock options and shares of restricted stock to employees, directors, officers and advisors of the company. For grants of stock options, the option exercise price is set at the closing price of the company's common stock on the date of grant, and the related number of shares underlying the stock option is fixed at that point in time. The Plan also provides for grants of restricted stock. Eligible employees may receive stock options annually with the number of shares and type of instrument generally determined by the employee's salary grade and performance level. In addition, certain management and professional level employees typically receive a stock option grant upon commencement of employment. The Plan does not allow key employees and directors (restricted persons) to exercise options during pre-defined blackout periods. Employees may participate in plans established pursuant to Rule 10b5-1 under the Exchange Act that allow them to exercise options according to pre-established criteria.

We use the Black-Scholes valuation model to estimate the grant date fair value of stock options and restricted stock. The expected volatility reflects the consideration of the historical volatility of our stock as determined by the closing price over a six to ten year term that is generally commensurate with the expected term of the award. Expected dividends reflect the quarterly distributions authorized and declared on our Class A and Class B common stock as of the grant date. The expected term of the awards are based on evaluations of historical and expected future employee exercise behavior. The risk-free interest rates for periods within the expected term of the award are based on the U.S. Treasury yield curve in effect during the period the options were granted. We use historical data to estimate future forfeiture rates to apply against the gross amount of compensation expense determined using the valuation model.

The weighted-average assumptions used to estimate the fair value of the stock options and restricted stock awards using the Black-Scholes valuation model were as follows for the three months ended March 31, 2014 and 2015:

Stock option information with respect to the company's stock-based equity plans during the three months ended March 31, 2015 is as follows:

Non-employee directors of the company have been awarded restricted stock awards that vest one year from the date of issuance. These restricted stock awards contained transfer restrictions under which they could not be sold, pledged, transferred or assigned until the sooner of the fifth anniversary from the grant date or the day after the non-employee director is no longer a member of the company's board. The restricted stock awards were independent of option grants and were granted at no cost to the recipient other than applicable taxes owed by the recipient. The awards were considered issued and outstanding from the date of grant.

The fair values of shares of restricted stock awards are determined based on the closing price of the company common stock on the grant dates. Information regarding the company's restricted stock awards during the three months ended March 31, 2015 is as follows:

The aggregate intrinsic value represents the difference between the company's closing stock price on March 31, 2015 of $6.16 and the option exercise price of the shares for stock options that were in the money, multiplied by the number of shares underlying such options. The total fair value of options vested during the three months ended March 31, 2014 and 2015 was $1.5 million and $1.3 million, respectively.

As of March 31, 2015, there was $0.7 million of total unrecognized compensation cost related to non-vested awards of stock.