Stock-based compensation expense reflects the fair value of stock-based awards measured at the grant date and recognized over the relevant service period. We estimate the fair value of each stock-based award on the measurement date using either the current market price of the stock or the Black-Scholes option valuation model. The Black-Scholes option valuation model incorporates assumptions as to stock price volatility, the expected life of options, a risk-free interest rate and dividend yield. We recognize stock-based compensation expense on a straight-line basis over the service period of the award, which is generally four to five years for options, and three years for restricted stock units.

The following table provides the classification of stock-based compensation as reflected in our consolidated statements of operations: