cash and cash equivalents cash equivalents include highly-liquid investments with a maturity of three months or less when purchased . accounts receivable and allowance for doubtful accounts accounts receivable are carried at the invoiced amounts , less an allowance for doubtful accounts , and generally do not bear interest . the company estimates the balance of allowance for doubtful accounts by analyzing accounts receivable balances by age and applying historical write-off and collection trend rates . the company 2019s estimates include separately providing for customer receivables based on specific circumstances and credit conditions , and when it is deemed probable that the balance is uncollectible . account balances are written off against the allowance when it is determined the receivable will not be recovered . the company 2019s allowance for doubtful accounts balance also includes an allowance for the expected return of products shipped and credits related to pricing or quantities shipped of $ 15 million , $ 14 million and $ 15 million as of december 31 , 2017 , 2016 , and 2015 , respectively . returns and credit activity is recorded directly to sales as a reduction . the following table summarizes the activity in the allowance for doubtful accounts: .
the company 2019s allowance for doubtful accounts balance also includes an allowance for the expected return of products shipped and credits related to pricing or quantities shipped . as of december 31 , 2017 , what percentage of the ending balance is this?
( a ) other amounts are primarily the effects of changes in currency translations and the impact of allowance for returns and credits . inventory valuations inventories are valued at the lower of cost or net realizable value . certain u.s . inventory costs are determined on a last-in , first-out ( 201clifo 201d ) basis . lifo inventories represented 39% ( 39 % ) and 40% ( 40 % ) of consolidated inventories as of december 31 , 2017 and 2016 , respectively . all other inventory costs are determined using either the average cost or first-in , first-out ( 201cfifo 201d ) methods . inventory values at fifo , as shown in note 5 , approximate replacement cost . property , plant and equipment property , plant and equipment assets are stated at cost . merchandising and customer equipment consists principally of various dispensing systems for the company 2019s cleaning and sanitizing products , dishwashing machines and process control and monitoring equipment . certain dispensing systems capitalized by the company are accounted for on a mass asset basis , whereby equipment is capitalized and depreciated as a group and written off when fully depreciated . the company capitalizes both internal and external costs of development or purchase of computer software for internal use . costs incurred for data conversion , training and maintenance associated with capitalized software are expensed as incurred . expenditures for major renewals and improvements , which significantly extend the useful lives of existing plant and equipment , are capitalized and depreciated . expenditures for repairs and maintenance are charged to expense as incurred . upon retirement or disposition of plant and equipment , the cost and related accumulated depreciation are removed from the accounts and any resulting gain or loss is recognized in income . depreciation is charged to operations using the straight-line method over the assets 2019 estimated useful lives ranging from 5 to 40 years for buildings and leasehold improvements , 3 to 20 years for machinery and equipment , 3 to 15 years for merchandising and customer equipment and 3 to 7 years for capitalized software . the straight-line method of depreciation reflects an appropriate allocation of the cost of the assets to earnings in proportion to the amount of economic benefits obtained by the company in each reporting period . depreciation expense was $ 586 million , $ 561 million and $ 560 million for 2017 , 2016 and 2015 , respectively. .