be a productive asset. Accordingly, we classify our DVD library in “Non-current content library” on the Consolidated Balance Sheets. The acquisition of DVD content library, net of changes in related liabilities, is classified in the line item “Acquisition of DVD content library” within cash used in investing activities in the Consolidated Statements of Cash Flows because the DVD content library is considered a productive asset. Other companies in the in-home entertainment video industry classify these cash flows as operating activities. We amortize our direct purchase DVDs, less estimated salvage value, on a “sum-of-the-months” accelerated basis over their estimated useful lives. The useful life of the new release DVDs and back-catalog DVDs is estimated to be one year and three years, respectively. The amortization of the DVD content library is classified in “Cost of revenues—Subscription” in the Consolidated Statement of Operations and in the line item “Amortization of DVD content library” within net cash provided by operating activities in the Consolidated Statements of Cash Flows. We also obtain DVD content through revenue sharing agreements with studios and distributors. Revenue sharing obligations incurred based on utilization are classified in “Cost of revenues—Subscription” in the Consolidated Statements of Operations and in the line item “Net income” within net cash provided by operating activities in the Consolidated Statements of Cash Flows. The terms of some revenue sharing agreements obligate us to make a low initial payment for certain titles, representing a minimum contractual obligation under the agreement. The low initial payment is in exchange for a commitment to share a percentage of our subscription revenues or to pay a fee, based on utilization, for a defined period of time. The initial payment may be in the form of an upfront non-refundable payment which is classified in content library or in the form of a prepayment of future revenue sharing obligations which is classified as prepaid content.

Stock-Based Compensation

Stock-based compensation expense at the grant date is based on the total number of options granted and an estimate of the fair value of the awards expected to vest and is recognized as expense ratably over the requisite service period, which is the vesting period.

We calculate the fair value of new stock-based compensation awards under our stock option plans using a lattice-binomial model. We use a Black-Scholes model to determine the fair value of employee stock purchase plan shares. These models require the input of highly subjective assumptions, including price volatility of the underlying stock. Changes in the subjective input assumptions can materially affect the estimate of fair value of options granted and our results of operations could be impacted.

 

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Expected Volatility: Our computation of expected volatility is based on a blend of historical volatility of our common stock and implied volatility of tradable forward call options to purchase shares of our common stock. Our decision to incorporate implied volatility was based on our assessment that implied volatility of publicly traded