Table:
|  | 2019 | 2018 |
| --- | --- | --- |
|  | Employee Grants | Employee Grants |
| Weighted-average Black-Scholes option pricing model assumptions: |  |  |
| Volatility | 73.01% | 82.00% |
| Expected term (in years) | 6 | 6 |
| Risk-free rate | 2.22% | 2.24% |
| Expected dividend yield | 0.0% | 0.0% |
| Weighted average grant date fair value per share | $2.10 | $2.22 |
Paragraphs:
The Company estimates the fair values of stock options using the Black-Scholes option-pricing model on the date of grant.For the years ended December 31, 2019 and 2018, the assumptions used in the Black-Scholes option pricing model, which was used to estimate the grant date fair value per option, were as follows:
The risk-free interest rate is the United States Treasury rate for the day of the grant having a term equal to the life of the equity instrument. The volatility is a measure of the amount by which the Company’s share price has fluctuated or is expected to fluctuate; the Company used its common stock volatility along with the average of historic volatilities of comparative companies. The dividend yield is zero as the Company has not made any dividend payment and has no plans to pay dividends in the foreseeable future. The Company determines the expected term of its stock option awards by using the simplified method, which assumes each vesting tranche of the award has a term equal to average of the contractual term and the vesting period.
As of December 31, 2019, total compensation cost not yet recognized related to unvested stock options was approximately $0.8 million, which is expected to be recognized over a weighted-average period of 2.3 years.
Question:
What are the respective risk-free rate of the 2018 and 2019 employee grants?
Answer: 2.24%, 2.22%