We provide a warranty for the replacement or repair of certain defective units, which varies based on the product sold.  Our standard warranty policy requires us to repair or replace certain defective units at no cost to our customers. We estimate the costs that may be incurred under our warranty and record an accrual in the amount of such costs at the time we recognize product revenue.  Factors that affect our warranty accrual for covered units include the number of units sold, historical and anticipated rates of warranty claims on these units, and the cost of such claims.  We periodically assess the adequacy of our recorded warranty accrual and adjust the amounts as necessary, which are included in accrued liabilities.

Changes in our warranty accrual are as follows: