for each separately vesting tranche. Compensation expense related to awards to non-employees with only service-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the associated service period of the award, which is generally the vesting term, using the Graded Vesting Attribution Method. Compensation expense related to awards to employees with only performance-based vesting conditions is recognized based on the estimated grant date fair value over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable. Compensation expense related to awards to non-employees only with performance-based vesting conditions is recognized based on the then-current fair value at each financial reporting date prior to the measurement date over the requisite service period using the Graded Vesting Attribution Method to the extent achievement of the performance condition is probable.
The Company estimates the fair value of its option awards to employees and directors using the Black-Scholes option-pricing model, which requires the input of and use of subjective assumptions, including (i) the fair value of the underlying common stock, (ii) the expected stock price volatility, (iii) the calculation of expected term of the award, (iv) the risk-free interest rate, and (v) expected dividends. Due to the lack of company-specific historical and implied volatility data of its common stock, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected term of the stock-based awards. The Company computes historical volatility data using the daily closing prices for the selected companies’ shares during the equivalent period of the calculated expected term of the stock-based awards. The Company’s estimates of expected term used in the Black-Scholes option pricing model were based on the estimated time from the grant date to the date of exercise. The risk-free interest rates for periods within the expected term of the option are based on the U.S. Treasury securities with a maturity date commensurate with the expected term of the associated award. The Company has never paid dividends, and does not expect to pay dividends in the foreseeable future.
Stock-based awards issued to non-employees, consisting of stock warrants and restricted common shares, are accounted for using the fair value method in accordance with ASC 505-50, Equity-Based Payments to Non-Employees. These stock warrants and restricted common shares have been granted in exchange for consulting services to be rendered, and vest according to certain service or performance conditions. In accordance with authoritative guidance, the fair value of non-employee stock-based awards is estimated on the date of grant, and subsequently revalued at each reporting period until the award vests or a measurement date has occurred using the Black-Scholes option-pricing model.
Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. There was no difference between net loss and comprehensive loss for each of the periods presented in the accompanying consolidated financial statements.
The Company is a C corporation for federal and state income tax purposes. The Company recognizes deferred tax assets and liabilities for temporary differences between the financial reporting basis and the tax basis of its assets and liabilities and the expected benefits of net operating loss carryforwards. The impact of changes in tax rates and laws on deferred taxes is applied in the period of enactment. The measurement of deferred tax assets is reduced if, based on weight of the evidence, it is more likely than not that some, or all, of the deferred tax assets will not be realized. At December 31, 2017, 2016 and 2015, the Company has concluded that as a result of the accumulated losses to date, a full valuation allowance is necessary for its net deferred tax assets.
The Company accounts for uncertain tax positions in accordance with the provisions of FASB ASC Topic 740, Income Taxes. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2017 and 2016, the Company does not have any significant uncertain tax