We have two active equity incentive plans, the "2013 Incentive Plan" and the "2011 Non-Employee Director Equity Incentive Plan", under which shares remain available for grants to employees and non-employee directors, respectively. In addition, we have made grants of inducement awards to certain executives and employees that are granted outside of, but governed by, the 2013 Incentive Plan. "Incentive stock options" under Section 422 of the U.S. Internal Revenue Code and restricted stock unit ("RSU") grants are part of our equity compensation practices for employees who receive equity grants. Options and RSUs generally vest quarterly over a four-year period beginning on the grant date. The contractual terms of options granted do not exceed ten years.

In May 2012, the Company's stockholders approved the 2012 Employee Stock Purchase Plan ("2012 ESPP"), which authorizes the issuance of 3.0 million shares of common stock to eligible employees to purchase shares of common stock through payroll deductions, which cannot exceed 10% of an employee's compensation. The purchase price of the shares is the lower of 85% of the fair market value of the stock at the beginning of each six-month offering period or 85% of the fair market value at the end of such period. We have treated the 2012 ESPP as a compensatory plan. At December 31, 2022, a total of 1.0 million shares of our common stock were available for future purchases under the 2012 ESPP.

At December 31, 2022, a total of 5.0 million shares of our common stock were available for future grants under the 2013 Incentive Plan and the 2011 Non-Employee Director Equity Incentive Plan. Following our 2018 Shareholder meeting, a share ratio of 2.2:1 was applied to the 2013 Incentive Plan. This ratio takes two and two-tenths shares out of the 2013 Plan for every one full value share granted. During fiscal 2022, a total of 2.5 million shares were adjusted out of the 2013 Plan. Shares subject to stock option grants that expire or are canceled, without delivery of such shares, generally become available for re-issuance under equity incentive plans.

Total stock-based compensation expense included in our Consolidated Statements of Operations is presented in the following table:

The fair values of the shares expected to be issued under the employee stock purchase plan and of each option award on the date of grant were estimated using the Black-Scholes valuation model and the assumptions noted in the following table. No new stock options were granted during fiscal 2022, 2021, or 2020. The expected volatility of both ESPP shares and stock options is based on the daily historical volatility of our stock price, measured over the ESPP purchase period or the expected term of the option. The risk-free interest rate is based on the implied yield on a U.S. Treasury zero-coupon issue with a remaining term closest to the expected term of the option. The expected term is based on historical vested option exercises and includes an estimate of the expected term for options that are fully vested and outstanding. Dividend yield has no valuation impact, as we have not paid any cash dividends since inception and do not intend to pay any cash dividends in the foreseeable future.

The following table summarizes the assumptions used in the valuation of ESPP compensation for the periods presented:

The weighted average fair values for the ESPP, calculated using the Black-Scholes option pricing model with the noted assumptions for the ESPP, were $15.25, $13.04, and $6.62 for fiscal years 2022, 2021, and 2020, respectively.

At December 31, 2022, there was no unrecognized compensation cost related to unvested employee and director stock options. Compensation expense for all stock-based compensation awards is recognized using the straight-line method. We recorded stock compensation expense related to the ESPP of approximately $1.5 million, $1.2 million, and $1.0 million in fiscal 2022, 2021, and 2020, respectively. Related to stock options, we recorded no expense in fiscal 2022, and approximately $1.0 million and $2.0 million, in fiscal 2021 and 2020, respectively.

The following table summarizes our stock option activity and related information for the year ended December 31, 2022:

The aggregate intrinsic value in the table above represents the total pretax intrinsic value (the difference between the Company's closing stock price on the last trading day of the fiscal year and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders had all option holders exercised their options on that day. This amount changes based on the fair market value of the Company's stock. Total intrinsic value of options exercised for fiscal 2022, 2021, and 2020 was $24.3 million, $44.7 million, and $21.5 million, respectively.

The following table summarizes the activity for our time-based RSUs for the year ended December 31, 2022:

At December 31, 2022, there was $81.6 million of unrecognized compensation expense related to unvested time-based RSUs. Compensation expense for RSUs is recognized using the straight-line method over the related vesting period. In fiscal 2022, 2021, and 2020, we recorded stock compensation expense related to time-based RSUs of approximately $30.1 million, $21.7 million, and $16.6 million, respectively.

In 2020 through 2022, we granted awards of RSUs with either a market condition or a performance condition to certain executives.

In the first quarters of fiscal 2022, 2021 and 2020, we granted awards of RSUs with a market condition to certain executives. Under the terms of these grants, the RSUs with a market condition vest over a three-year period based on the Company’s total shareholder return ("TSR") relative to the Russell 2000 index, which condition is measured for the 2022 and 2021 grants on the third anniversary of the grant date, and measured for one-half of the 2020 grants on the second and third anniversary of the grant date. The awards may vest at 250% or 200%, depending upon the executive, if the 75th percentile of the market condition is achieved, with 100% of the units vesting at the 55th percentile, zero vesting if relative TSR is below the 25th percentile, and vesting scaling for achievement between the 25th and 75th percentile.

In the first quarter of fiscal 2021, we also granted awards of RSUs with a performance condition to certain executives, to specifically drive additional executive attention and focus on the Company’s revenue growth priorities. Under the terms of these grants, the RSUs with a performance condition will vest based on the Company generating specified levels of year-over-year revenue growth, which will be measured annually for one-fourth of the grants after each fiscal year-end through the end of fiscal 2024. Vesting of these awards scales for achievement of year-over-year revenue growth compared to certain targets, with maximum vesting up to 200%. Vesting of each tranche of these awards occurs 13 months after the performance conditions is met, and the entire award cannot be fully earned until five years from grant. For the first and second tranches of these awards, the Company had met the year-over-year revenue growth performance criteria at the 200% level of achievement as of January 1, 2022 and December 31, 2022, respectively.

During fiscal 2022, the market condition for awards granted to certain executives in previous years exceeded the 75th percentile of their TSR condition, and applicable tranches of these awards vested at 250% or 200% for the respective executives. Also during fiscal 2022, the fifth and sixth tranches of 40% and 70%, respectively, of the base number of the awards with an EBITDA performance condition vested, as the Company had met the adjusted EBITDA performance criteria on a trailing four-quarter basis for two consecutive trailing four-quarter periods as of the end of the respective measurement periods.

During fiscal 2021, the market condition for awards granted to certain executives in previous years exceeded the 75th percentile of their TSR condition, and applicable tranches of these awards vested at 250% or 200% for the respective executives. Also during fiscal 2021, the second and third tranches, each 33.3% of the base number of the awards with an EBITDA performance condition vested and released, as the Company had met the adjusted EBITDA performance criteria on a trailing four-quarter basis for two consecutive trailing four-quarter periods as of the end of the respective measurement periods. Additionally, as of January 1, 2022, the Company had met the next adjusted EBITDA performance criteria on a trailing four-quarter basis for two consecutive trailing four-quarter periods, and the fourth tranche of the awards with an EBITDA performance condition qualified for vesting at 40% of the base number.

During fiscal 2020, the market condition for awards granted to certain executives in previous years exceeded the 75th percentile of their TSR condition, and annual tranches of these awards vested at 250% or 200%, and applicable tranches of these awards vested at 250% or 200% for the respective executives. Also during fiscal 2020, the first tranche of 33.3% of the base number of the awards with an EBITDA performance condition vested, as the Company had met the adjusted EBITDA performance criteria on a trailing four-quarter basis for two consecutive trailing four-quarter periods as of the end of the measurement period.

Market-Based and Performance-Based Awards — Compensation Expense

During the first quarter of fiscal 2020, the Board of Directors approved a modification to the market condition measurement periods associated with the unvested portions of certain of the Company’s awards with a market condition that were granted prior to fiscal 2020. The modification extended the duration of the measurement period by adjusting the beginning date of each measurement period to the original grant date, resulting in approximately $1.8 million additional stock compensation expense during the first quarter of fiscal 2020.

For our awards with a market condition or a performance condition, we incurred stock compensation expense, including the effect of the modification in the first quarter of fiscal 2020, of approximately $24.0 million, $22.1 million, and $20.8 million in fiscal years 2022, 2021, and 2020, respectively, which is recorded as a component of total stock-based compensation. At December 31, 2022, there was $21.6 million of unrecognized compensation expense related to unvested RSUs with a market condition or a performance condition. Awards with a TSR market condition were valued using a Monte Carlo simulation model.

The following table summarizes the assumptions used at the grant date in the valuation of RSUs with a market or performance condition:

The following table summarizes the activity for our awards with a market condition or performance condition: