and incentives, and product returns. When we cannot reasonably estimate the amount of future product returns, we record revenues when the risk of product return has been substantially eliminated. We record sales of certain of our vaccines to the U.S. government as part of the Pediatric Vaccine Stockpile program; these rules require that for fixed commitments made by the U.S. government, we record revenues when risk of ownership of the completed product has been passed to the U.S. government. There are no specific performance obligations associated with products sold under this program.

Deductions from Revenues—As is typical in the biopharmaceutical industry, our gross product sales are subject to a variety of deductions that generally are estimated and recorded in the same period that the revenues are recognized and primarily represent rebates and discounts to government agencies, wholesalers, distributors and managed care organizations with respect to our biopharmaceutical products. These deductions represent estimates of the related obligation and, as such, judgment and knowledge of market conditions and practice are required when estimating the impact of these sales deductions on gross sales for a reporting period.

Specifically,

• In the U.S., we record provisions for pharmaceutical Medicaid, Medicare and contract rebates based upon our experience ratio of rebates paid and actual prescriptions written during prior quarters. We apply the experience ratio to the respective period’s sales to determine the rebate accrual and related expense. This experience ratio is evaluated regularly to ensure that the historical trends are as current as practicable. As appropriate, we will adjust the ratio to better match our current experience or our expected future experience. In assessing this ratio, we consider current contract terms, such as changes in formulary status and discount rates. If our ratio is not indicative of future experience, our results could be materially affected.

• Outside the U.S., the majority of our pharmaceutical rebates, discounts and price reductions are contractual or legislatively mandated, and our estimates are based on actual invoiced sales within each period; both of these elements help to reduce the risk of variations in the estimation process. Some European countries base their rebates on the government’s unbudgeted pharmaceutical spending, and we use an estimated allocation factor (based on historical payments) and total revenues by country against our actual invoiced sales to project the expected level of reimbursement. We obtain third-party information that helps us monitor the adequacy of these accruals. If our estimates are not indicative of actual unbudgeted spending, our results could be materially affected.

• Provisions for pharmaceutical chargebacks (primarily reimbursements to wholesalers for honoring contracted prices to third parties) closely approximate actual as we settle these deductions generally within two to five weeks of incurring the liability.

• Provisions for pharmaceutical returns are based on a calculation in each market that incorporates the following, as appropriate: local returns