Years ended December 31: | 2017 | 2016 | 2015 | |||||||||
Expected option life | 4.5 years | 4.8–5.0 years | 1.0–5.3 years | |||||||||
Risk free interest rate | 1.71 | % | 0.52%-0.67% | 0.57%-0.92% | ||||||||
Dividend yield | nil | nil | nil | |||||||||
Expected volatility | 80.44 | % | 115.59%-117.56% | 135.03%-171.07% |