| | 2017 | | 2016 | | 2015 | | |||
Risk-free interest rate | | | 0.78% - 1.74 | % | | - | | | 0.78% - 1.68 | % |
Dividend yield | | | 0 | % | | - | | | 0 | % |
Volatility | | | 97 | % | | - | | | 86 | % |
Forfeiture rate | | | 21 | % | | - | | | 23 | % |
Estimated expected lives | | | 5 years | | | - | | | 5 years | |