Note 15. Stock-Based Compensation
The following table summarizes the components of non-cash stock-based compensation expense (in thousands):
2015 Equity Incentive Plan
We issue stock options pursuant to our 2015 Plan. The 2015 Plan allows for the grant of stock options to employees and for the grant of nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, or RSUs, performance-based stock awards, and other forms of equity compensation to our employees, directors and non-employee directors and consultants.
In June 2015, our board of directors adopted and our stockholders approved our 2015 Plan pursuant to which we initially reserved a total of 4,700,000 shares of common stock for issuance under the 2015 Plan, which included shares of our common stock previously reserved for issuance under our Amended and Restated 2009 Stock Incentive Plan, or the 2009 Plan. The number of shares of common stock reserved for issuance under the 2015 Plan will automatically increase on January 1 each year, for a period of not more than ten years, commencing on January 1, 2016 through January 1, 2024, by 5% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the board of directors. As a result of the adoption of the 2015 Plan, no further grants may be made under the 2009 Plan. As of December 31, 2019, 6,527,550 shares remained available for future grant under the 2015 Plan.
Stock Options
Stock options under the 2015 Plan have been granted at exercise prices based on the closing price of our common stock on the date of grant. Stock options under the 2009 Plan were granted at exercise prices as determined by the board of directors to be the fair market value of our common stock. Our stock options generally vest over a five-year period and each option, if not exercised or forfeited, expires on the tenth anniversary of the grant date.
Certain stock options granted under the 2015 Plan and previously granted under the 2009 Plan may be exercised before the options have vested. Unvested shares issued as a result of early exercise are subject to repurchase by us upon termination of employment or services at the original exercise price. The proceeds from the early exercise of stock options are initially recorded as a current liability and are reclassified to common stock and additional paid-in capital as the awards vest and our repurchase right lapses. There were 250 and 957 unvested shares of common stock outstanding subject to our right of repurchase as of December 31, 2019 and 2018, respectively. We repurchased 27 and 107 of these unvested shares of common stock related to early exercised stock options in connection with employee terminations during the years ended December 31, 2019 and 2018, respectively. We recorded less than $0.1 million in accounts payable, accrued expenses and other current liabilities on our consolidated balance sheets for the proceeds from the early exercise of the unvested stock options as of December 31, 2019 and 2018.
We account for stock-based compensation options based on the fair value of the award as of the grant date. We recognize stock-based compensation expense using the accelerated attribution method, net of actual forfeitures, in which compensation cost for each vesting tranche in an award is recognized ratably from the service inception date to the vesting date for that tranche.
We value our stock options using the Black-Scholes option pricing model, which requires the input of subjective assumptions, including the risk-free interest rate, expected term, expected stock price volatility and dividend yield. The risk-free interest rate assumption is based upon observed interest rates for constant maturity U.S. Treasury securities consistent with the expected term of our stock options. The expected term represents the period of time the stock options are expected to be outstanding and is based on the "simplified method." Under the "simplified method," the expected term of an option is presumed to be the mid-point between the vesting date and the end of the contractual term. We use the "simplified method" due to the lack of sufficient historical exercise data to provide a reasonable basis upon which to otherwise estimate the expected term of the stock options. Beginning in November 2019, the expected volatility for options granted is based on historical volatilities of our stock over the estimated expected term of the stock options. The expected volatility for options granted prior to November 2019 was based on historical volatilities of our stock and publicly traded stock of comparable companies over the estimated expected term of the stock options.
There were 186,500, 219,450 and 252,100 stock options granted during the years ended December 31, 2019, 2018 and 2017, respectively. We declared and paid dividends in June 2015 in anticipation of our IPO, which we closed on July 1, 2015. Subsequent to the IPO, we do not expect to declare or pay dividends on a recurring basis. As such, we assume that the dividend rate is 0%.
Table:
 |  | Year Ended December 31, | 
 | 2019 | 2018 | 2017
Stock options and assumed options | $3,783 | $3,511 | $3,913
Restricted stock units | 16,627 | 9,770 | 3,366
Restricted stock awards | — | 1 | 19
Employee stock purchase plan | 193 | 147 | 115
Total stock-based compensation expense | $20,603 | $13,429 | $7,413
Tax benefit from stock-based awards | $5,154 | $7,581 | $12,719


Question: What was the percentage change in the Total stock-based compensation expense between 2018 and 2019?
Answer: 53.42
Calculation: (20,603-13,429)/13,429