NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (continued)
restricted stock units (“RSUs”), based on the grant-date fair value of the award and recognize the cost of such awards over the requisite service period (generally, the vesting period of the award). ASC 718 requires that an entity measure the cost of liability-based service awards based on current fair value that is remeasured subsequently at each reporting date through the settlement date. The compensation expense associated with performance stock units (“PSUs”) is recognized ratably over the period between when the performance conditions are deemed probable of achievement and when the awards are vested. Performance stock options (“PSOs”) are valued and stock-based compensation expense is recorded once the performance conditions of the outstanding PSOs have achieved probability. Prior to July 1, 2018, we accounted for equity awards issued to non-employees in accordance with ASC 505-50, Equity-Based Payments to Non-Employees; see “k) Recent Accounting Pronouncements” for further details related to our adoption of ASU 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, during the three months ended September 30, 2018 and our current accounting for equity awards issued to non-employees.
We have historically recorded stock-based compensation expense based on the number of stock options or RSUs we expect to vest using our historical forfeiture experience and we periodically update those forfeiture rates to apply to new grants. While we early adopted Accounting Standards Update (“ASU”) 2016-09, Improvements to Employee Share-Based Payment Accounting during the year ended December 31, 2016, we have elected to continue to estimate forfeitures under the true-up provision of ASC 718. We record additional expense if the actual forfeiture rate is lower than estimated, and record a recovery of prior expense if the actual forfeiture rate is higher than estimated.
We estimate the fair value of our stock options using the Black‑Scholes option valuation model. The Black‑Scholes option valuation model requires the use of certain subjective assumptions. The most significant of these assumptions are the estimates of the expected volatility of the market price of our stock and the expected term of the award. We base our estimates of expected volatility on the historical average of our stock price. The expected term represents the period of time that share‑based awards granted are expected to be outstanding. Other assumptions used in the Black‑Scholes option valuation model include the risk‑free interest rate and expected dividend yield. The risk‑free interest rate for periods pertaining to the expected term of each option is based on the U.S. Treasury yield of a similar duration in effect at the time of grant. We have never paid, and do not expect to pay, dividends in the foreseeable future.
We estimate the fair value of our PSUs using a Monte Carlo simulation. This model uses assumptions, including the risk free interest rate, expected volatility of our stock price and those of the performance group, dividends of the performance group members and expected life of the awards. As noted above, we continue to estimate forfeitures under the true-up provision of ASC 718. If it becomes probable that the PSU performance targets will be met, compensation expense will be recorded for these awards ratably over the requisite service period. The PSUs are forfeited to the extent the performance criteria are not met.
h) Income Taxes
We account for income taxes under the liability method, as described in ASC 740 - Income Taxes (“ASC 740”). Deferred income taxes are recognized for the tax consequences of temporary differences between the tax and financial statement reporting bases of assets and liabilities. When we determine that we will not be able to realize our deferred tax assets, we adjust the carrying value of the deferred tax asset through the valuation allowance.