The following tables set forth activity and various statistics related to our performance stock unit awards and the assumptions used by us in the valuations of the 2019, 2018 and 2017 grants at the grant date: | | | | | | | | | | | Number of Units | | Weighted- Average Price | Nonvested at December 31, 2018 | | 219,331 |
| | $ | 69.21 |
| Granted | | 71,237 |
| | $ | 89.86 |
| Vested | | (70,548 | ) | | $ | 64.06 |
| Forfeited | | (1,844 | ) | | $ | 73.57 |
| Nonvested at December 31, 2019 | | 218,176 |
| | $ | 77.58 |
|
| | | | | | | | | | | 2019 | | 2018 | | 2017 | | Volatility (a) | | 18.70% | | 18.80% | | 20.70% | | Dividend yield | | 2.38% | | 2.70% | | 2.63% | | Risk-free interest rate (b) | | 2.50% | | 2.38% | | 1.48% | |
(a) - Volatility based on historical volatility over three years using daily stock price observations of our peer utilities. (b) - Using 3-year treasury. | | | | | | | | | | | | | | | | 2019 | | 2018 | | 2017 | Weighted-average grant date fair value (per share) | | $ | 89.86 |
| | $ | 74.04 |
| | $ | 68.94 |
| Fair value of shares granted (thousands of dollars) | | $ | 6,401 |
| | $ | 5,882 |
| | $ | 5,110 |
|
|