equal to the entire difference between the debt instruments’ amortized cost basis and its fair value. For available-for-sale debt instruments that are considered other-than-temporarily impaired due to the existence of a credit loss, if we do not intend to sell and it is more likely than not that we will not be required to sell the instrument before recovery of its remaining amortized cost basis (amortized cost basis less any current-period credit loss), we separate the amount of the impairment into the amount that is credit related and the amount due to all other factors. The credit loss component is recognized in earnings.



We performed an impairment review of our investment portfolio as of January 25, 2015. We concluded that our investments were appropriately valued and that no other than temporary impairment charges were necessary on our portfolio of available-for-sale investments as of January 25, 2015.



Stock-based Compensation



Our stock-based compensation expense is associated with stock options, restricted stock units, or RSUs, performance stock units, or PSUs, and our employee stock purchase plan, or ESPP.



During fiscal year 2015, we shifted away from granting stock options and toward granting RSUs and PSUs to reflect changing market trends for equity incentives at our peer companies. The number of PSUs that will ultimately vest is contingent on the Company’s level of achievement compared with the corporate financial performance target established by our Compensation Committee in the beginning of each fiscal year. The number of shares of our stock to be received at vesting ranges from 0% to 200% of the target amount.



We use the closing trading price of our common stock on the date of grant, minus a dividend yield discount, as the fair value of awards of RSUs and PSUs. The compensation expense for RSUs is recognized using a straight-line attribution method over the requisite employee service period, while compensation expense for PSUs is recognized using an accelerated amortization model. We estimate the fair value of shares to be issued under our ESPP using the Black-Scholes model at the commencement of an offering period in March and September of each year. Stock-based compensation for our ESPP is expensed using an accelerated amortization model.



Our RSU and PSU awards are not eligible for cash dividends prior to vesting; therefore, the fair value of RSUs and PSUs is discounted by the dividend yield. Additionally, we estimate forfeitures annually based on historical experience and revise the estimates of forfeiture in subsequent periods if actual forfeitures differ from those estimates. If factors change, the compensation expense that we record under these accounting standards may differ significantly from what we have recorded in the current period.



Litigation, Investigation and Settlement Costs



From time to time, we are involved in legal actions and/or investigations by regulatory bodies. We are aggressively defending our current litigation matters. However, there are many uncertainties associated with any litigation or investigations, and we cannot be certain that these actions or other third-party claims