Summarized financial information for the Partnership's reportable segments is presented in the table below: Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Net sales: Refined products $ 515,021 $ 551,489 $ 2,219,457 $ 2,396,374 Natural gas 48,987 46,908 221,262 235,263 Materials handling 13,119 14,711 43,913 42,077 Other operations 5,463 5,347 18,284 17,545 Net sales $ 582,590 $ 618,455 $ 2,502,916 $ 2,691,259 Adjusted gross margin (1): Refined products $ 33,400 $ 26,646 $ 105,783 $ 111,652 Natural gas 3,681 3,007 40,649 46,010 Materials handling 13,101 14,683 43,886 42,100 Other operations 1,565 1,715 5,146 5,496 Adjusted gross margin 51,747 46,051 195,464 205,258 Reconciliation to operating income (loss) (2): Add/(deduct): Change in unrealized gain on inventory (3) 3,428 (3,281 ) (1,169 ) 19,309 Change in unrealized value on natural gas transportation contracts (4) (7,005 ) (5,939 ) 6,429 4,413 Operating costs and expenses not allocated to operating segments: Operating expenses (20,461 ) (21,047 ) (65,325 ) (66,537 ) Selling, general and administrative (17,570 ) (16,923 ) (56,309 ) (63,349 ) Depreciation and amortization (8,466 ) (8,343 ) (25,263 ) (25,146 ) Operating income (loss) 1,673 (9,482 ) 53,827 73,948 Other income  293 128 293 Interest income 121 123 447 404 Interest expense (9,918 ) (9,073 ) (31,915 ) (28,369 ) Income tax provision (1,610 ) (295 ) (3,078 ) (2,984 ) Net (loss) income $ (9,734 ) $ (18,434 ) $ 19,409 $ 43,292 (1) The Partnership trades, purchases, stores and sells energy commodities that experience market value fluctuations. To manage the Partnerships underlying performance, including its physical and derivative positions, management utilizes adjusted gross margin, which is a non-GAAP financial measure. Adjusted gross margin is also used by external users of the Partnerships consolidated financial statements to assess the Partnerships economic results of operations and its commodity market value reporting to lenders. In determining adjusted gross margin, the Partnership adjusts its segment results for the impact of unrealized gains and losses with regard to refined products and natural gas inventory, and natural gas transportation contracts, which are not marked to market for the purpose of recording unrealized gains or losses in net income. (2) Reconciliation of adjusted gross margin to operating income, the most directly comparable GAAP measure. (3) Inventory is valued at the lower of cost or net realizable value. The adjustment related to change in unrealized gain on inventory which is not included in net income, represents the estimated difference between inventory valued at the lower of cost or net realizable value as compared to market values. The fair value of the derivatives the Partnership uses to economically hedge its inventory declines or appreciates in value as the value of the underlying inventory appreciates or declines, which creates unrealized hedging losses (gains) with respect to the derivatives that are included in net income. (4) Represents the Partnerships estimate of the change in fair value of the natural gas transportation contracts which are not recorded in net income until the transportation is utilized in the future (i.e., when natural gas is delivered to the customer), as these contracts are executory contracts that do not qualify as derivatives. As the fair value of the natural gas transportation contracts decline or appreciate, the offsetting physical or financial derivative will also appreciate or decline creating unmatched unrealized gains (losses).