The effect of recording stock-based compensation expense for the years ended December 31, 2020, 2019 and 2018 is as follows (in thousands):

	Years Ended December 31,
	2020			2019			2018
Cost of licensing, services and software revenue, excluding depreciation and amortization of intangible assets	$	781		$	—		$	—
Research, development and other related costs		13,592			14,643			13,168
Selling, general and administrative		24,762			16,911			17,843
Total stock-based compensation expense		39,135			31,554			31,011
Tax effect on stock-based compensation expense		(2,116	)		(5,051	)		(4,869	)
Net effect on net income (loss)	$	37,019		$	26,503		$	26,142

In connection with the Mergers, the Company assumed unvested TiVo equity awards with a fair value of $34.1 million, of which $22.0 million related to post-acquisition services. For the year ended December 31, 2020, the Company recognized $5.8 million associated with the unvested TiVo equity awards assumed. See “Note 9 – Business Combination” for additional detail.

Stock-based compensation expense categorized by various equity components for the years ended December 31, 2020, 2019 and 2018 is summarized in the table below (in thousands):

	Years Ended December 31,
	2020		2019		2018
Restricted stock awards and units	$	36,451	$	29,031	$	27,974
Employee stock purchase plan		2,613		2,304		2,599
Employee stock options		71		219		438
Total stock-based compensation expense	$	39,135	$	31,554	$	31,011

The total fair value of restricted stock awards and units vested during the years ended December 31, 2020, 2019 and 2018 was $45.7 million, $30.1 million and $30.4 million, respectively.

The total intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 was $0.1 million, $0.2 million and $1.0 million, respectively. The intrinsic value is calculated as the difference between the market value on the date of exercise and the exercise price of the shares.

As of December 31, 2020, the unrecognized stock-based compensation balance after estimated forfeitures consisted of $0.1 million related to unvested stock options, to be recognized over an estimated weighted average amortization period of 0.8 years, and $73.5 million related to restricted stock awards and units, including performance-based awards and units, to be recognized over an estimated weighted average amortization period of 2.7 years.

As of December 31, 2019, the unrecognized stock-based compensation balance after estimated forfeitures consisted of $0.1 million related to unvested stock options, to be recognized over an estimated weighted average amortization period of 1.5 years, and $40.3 million related to restricted stock awards and units, including performance-based awards and units, to be recognized over an estimated weighted average amortization period of 2.3 years.

The Company uses the Black-Scholes option pricing model to determine the estimated fair value of options and ESPP shares. The fair value of each option grant is determined on the date of grant and the expense is recorded on a straight-line basis. The assumptions used in the model include expected life, volatility, risk-free interest rate, and dividend yield. The Company’s determinations of these assumptions are outlined below.

Expected life – The expected life assumption is based on analysis of the Company’s historical employee exercise patterns. The expected life of options granted under the ESPP represents the offering period of two years.

Volatility – Volatility is calculated using the historical volatility of the Company’s common stock for a term consistent with the expected life. Historical volatility of the Company’s common stock is also utilized for the ESPP.

Risk-free interest rate – The risk-free interest rate assumption is based on the U.S. Treasury rate for issues with remaining terms similar to the expected life of the options.

Dividend yield – Expected dividend yield is calculated based on cash dividends declared by the Board for the previous four quarters and dividing that result by the average closing price of the Company’s common stock for the quarter. Cash dividends are not paid on options, restricted stock units or unvested restricted stock awards.

In addition, the Company estimates forfeiture rates. Forfeitures are estimated at the time of grant and revised in subsequent periods if actual forfeitures differ from those estimates. Historical data is used to estimate pre-vesting option forfeitures and record stock-based compensation expense only for those awards that are expected to vest.

There were no stock options granted during the years ended December 31, 2020, 2019 and 2018.

	Years Ended December 31,
	2020		2019		2018
Expected life (years)		2.0		2.0		2.0
Risk-free interest rate	0.1 - 1.4%		1.7 - 2.5%		2.2 - 2.7%
Dividend yield	1.4 - 4.0%		3.5 - 5.4%		3.6 - 3.9%
Expected volatility	45.8 - 57.5%		51.7 - 53.4%		39.4 - 41.7%

For the years ended December 31, 2020, 2019 and 2018, an aggregate of 355,000, 386,000 and 306,000 common shares, respectively, were purchased pursuant to the ESPP.

The Company uses a Monte Carlo simulation to determine the grant date fair value of performance stock units subject to market conditions, or market-based PSUs. The following assumptions were used to value the performance stock units subject to market conditions granted in the year ended December 31, 2020:

	Year Ended December 31, 2020
Expected life (years)		3.0
Risk-free interest rate		0.2	%
Dividend yield		1.4	%
Expected volatility		51.3	%

There were no market-based PSUs granted during the years ended December 31, 2019 and 2018.

From time to time, the Company enters into consulting agreements with its departing employees. Some of these agreements may include continued vesting of the departing employees’ stock awards and an extension of the exercise period from the standard 90 days from employment termination date to the termination of the consulting agreement. There were no modifications in the years ended December 31, 2020, 2019 and 2018, respectively.