12. Stock-Based Compensation Expense

For stock-based awards granted by the Company, stock-based compensation expense is measured at grant date based on the fair value of the award and is expensed over the requisite service period. The Company recorded stock-based compensation expense of $15,619, $9,242 and $5,218 for 2015, 2014 and 2013, respectively.

Stock Options

The Company uses the Black-Scholes option pricing model to estimate the fair value of options. This model requires the input of highly subjective assumptions including the expected volatility, risk-free interest rate, and the expected life of options. The Company used the following assumptions:

	Years Ended December 31,
	2015			2014			2013
Dividend yield		—			—			—
Expected volatility		49.1	%		51.3	%		55.0	%
Risk-free interest rate		1.75	%		1.89	%		1.27	%
Expected life of options (in years)		6.25			6.25			6.25
Forfeiture rate		7.0	%		7.0	%		7.0	%
Weighted-average grant-date fair value	$	2.88		$	4.77		$	6.41
Weighted-average grant-date exercise price	$	5.85		$	9.39		$	11.98

As the Company has limited historical option exercise data, the expected term of the stock options granted to employees was calculated based on the simplified method. Under the simplified method, the expected term is equal to the average of an option’s weighted-average vesting period and its contractual term. Pursuant to the SEC Staff Accounting Bulletin (“SAB”) No. 110, the Company is permitted to continue using the simplified method until sufficient information regarding exercise behavior, such as historical exercise data or exercise information from external sources, becomes available. The Company estimates the expected volatility of its common stock on the date of grant based on the historical stock volatilities of similar publicly-traded entities over a period equal to the expected terms of the options, as the Company does not have sufficient trading history to use the volatility of its own common stock. The Company has no history or expectation of paying cash dividends on its common stock. The risk-free interest rate is based on the United States Treasury yield for a term consistent with the expected life of the options in effect at the time of grant.

Cash proceeds from the exercise of stock options were $1,439, $2,472, and $1,541 during 2015, 2014 and 2013, respectively.

Compensation expense is recognized ratably over the requisite service period. As of December 31, 2015 and 2014, there was $10,204 and $16,112, respectively, of unrecognized compensation cost related to options, which is expected to be recognized over a weighted-average period of 2.6 and 3.0 years, respectively.

Prior to the consummation of the IPO, given the lack of an active public market for the Company’s outstanding Common and preferred stock, the Company’s Board of Directors established an estimate of fair value for these securities as well as for options and warrants to purchase these securities. The fair value of the Company’s common stock as used in the determination of the exercise price of stock options was estimated by the Board of Directors based on factors such as the liquidation preference, dividends and other rights of the outstanding preferred stock; recent financial and operating performance; the status of the Company’s development and sales efforts, revenue growth and additional objectives; the likelihood and proximity of an IPO; and the valuation of comparable companies that are publicly traded. Subsequent to the closing of the Company’s IPO, the Company has used the closing price of the Company’s common stock on the date of the stock option grant as the fair value of the Company’s common stock and the exercise price of the stock options.

For awards that are expected to result in a tax deduction, a deferred tax asset is established as the Company recognizes compensation expense. If the tax deduction exceeds the cumulative recorded compensation expense, the tax benefit associated with the excess deduction is considered a windfall benefit. The excess tax benefit from share compensation plans is recorded in additional paid-in capital and classified as a financing cash flow on the consolidated statements of cash flows. The Company has elected to use the “with and without” approach as described in ASU 740-20, Intraperiod Tax Allocation, in determining the order in which tax attributes are utilized. As a result, the Company will only recognize a tax benefit from stock-based awards in additional paid-in capital if an incremental tax benefit is realized after all other tax attributes currently available to the Company have been utilized.

Restricted Stock and RSUs

As of December 31, 2015 and 2014, there was $10,667 and $9,459 of unrecognized compensation cost related to restricted stock and RSUs, respectively, which is expected to be recognized over a weighted-average period of 2.0 and 2.2 years, respectively. The Company uses the fair market value of the underlying common stock on the dates of grant to determine the fair value of restricted stock and RSUs. Stock-based compensation expense related to these awards is recognized on a straight-line basis over the service period of the award for the estimated number of shares that are ultimately expected to vest.

Employee Stock Purchase Plan

The Company estimates the fair value of purchase rights under the ESPP using the Black-Scholes valuation model. The fair value of each purchase right under the ESPP was estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with assumptions substantially similar to those used for the valuation of our stock option awards.