We use the Black-Scholes options pricing model to estimate the fair value of our stock option grants with only service conditions and the fair value of our common stock under the 2012 ESPP. This model incorporates various assumptions including expected volatility, expected term, risk-free interest rates and expected dividend yields. The following assumptions were used to calculate our stock-based compensation for each stock option grant on the date of the grant:



	Year Ended
	December 31, 2017
Stock Options with only service conditions:
Expected volatility	39% - 42%
Expected term (in years)	4.89
Risk-free interest rate	1.78% - 2.47%
Dividend yield	—	%

Apart from the options granted to our Chief Executive Officer as described in Note 14, which have both service and market-based vesting conditions, no other stock options were granted during the years ended December 31, 2019 and 2018.

The following assumptions were used in the Black-Scholes options pricing model to calculate our stock-based compensation for each stock purchase right granted under the 2012 ESPP:



	Year Ended December 31,
	2019		2018		2017
ESPP:
Expected volatility	30% - 49%		26% - 31%		28% - 49%
Expected term (in years)	0.50		0.50		0.50
Risk-free interest rate	2.04% - 2.46%		1.15% - 2.22%		0.40% - 1.15%
Dividend yield	—	%	—	%	—	%

Expected volatility. The expected volatility is based on the historical volatility of our common stock for a period similar to our expected term.

Expected term. We determine the expected term for stock options based on historical experience of similar awards, giving consideration to the contractual terms of the stock-based awards, vesting schedules and expectations of future employee behavior. We estimate the expected term for ESPP using the purchase period.

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant for the expected term of the stock-based award.

Expected dividend yield. Our expected dividend yield is zero, as we have not and do not currently intend to declare dividends in the foreseeable future.