Segment Information The Company has three reportable segments: Corporate, which is comprised primarily of US private sector business customers with more than 250 employees, Small Business, which primarily serves US private sector business customers with up to 250 employees, and Public, which is comprised of government agencies and education and healthcare institutions in the US. The Company has two other operating segments: CDW UK and CDW Canada, both of which do not meet the reportable segment quantitative thresholds and, accordingly, are included in an all other category ("Other"). Headquarters function costs that are not allocated to the segments are included under the heading of "Headquarters" in the tables below. During the first quarter of 2019, the Company evaluated its methodology for allocating certain depreciation and amortization expenses to each of its segments. The evaluation resulted in a revision to the allocation of depreciation and amortization expenses from Headquarters to the Companys reportable segments, effective January 1, 2019. The prior period results have been recast to reflect these changes and present comparable information. Information about the Company's segments is as follows: (in millions) Corporate Small Business Public Other Headquarters Total Three Months Ended September 30, 2019 Net sales $ 1,913.5 $ 386.2 $ 2,100.9 $ 507.1 $  $ 4,907.7 Operating income (loss) 138.9 27.2 168.4 21.9 (35.8 ) 320.6 Depreciation and amortization expense (21.7 ) (5.6 ) (15.0 ) (7.9 ) (18.0 ) (68.2 ) Three Months Ended September 30, 2018 Net sales $ 1,706.5 $ 340.0 $ 1,875.1 $ 451.6 $  $ 4,373.2 Operating income (loss) 125.7 23.8 136.6 19.2 (30.5 ) 274.8 Depreciation and amortization expense (21.8 ) (5.5 ) (12.9 ) (7.3 ) (18.3 ) (65.8 ) (in millions) Corporate Small Business Public Other Headquarters Total Nine Months Ended September 30, 2019 Net sales $ 5,533.6 $ 1,119.2 $ 5,271.7 $ 1,571.0 $  $ 13,495.5 Operating income (loss) 430.8 78.2 373.9 66.6 (99.7 ) 849.8 Depreciation and amortization expense (64.8 ) (16.8 ) (41.4 ) (23.1 ) (51.4 ) (197.5 ) Nine Months Ended September 30, 2018 Net sales $ 5,006.1 $ 997.1 $ 4,740.5 $ 1,422.0 $  $ 12,165.7 Operating income (loss) 388.3 70.0 318.3 60.7 (92.9 ) 744.4 Depreciation and amortization expense (66.8 ) (16.6 ) (38.4 ) (25.1 ) (51.8 ) (198.7 ) Geographic Areas and Revenue Mix Three Months Ended September 30, 2019 (in millions) Corporate Small Business Public Other Total Geography (1) United States $ 1,910.9 $ 386.2 $ 2,100.9 $ 7.6 $ 4,405.6 Rest of World 2.6   499.5 502.1 Total Net sales 1,913.5 386.2 2,100.9 507.1 4,907.7 Major Product and Services Hardware 1,528.0 326.6 1,711.3 358.3 3,924.2 Software 269.1 47.6 324.4 66.1 707.2 Services 97.9 6.7 59.0 80.2 243.8 Other (2) 18.5 5.3 6.2 2.5 32.5 Total Net sales 1,913.5 386.2 2,100.9 507.1 4,907.7 Sales by Channel Corporate 1,913.5    1,913.5 Small Business  386.2   386.2 Government   793.4  793.4 Education   807.0  807.0 Healthcare   500.5  500.5 Other    507.1 507.1 Total Net sales 1,913.5 386.2 2,100.9 507.1 4,907.7 Timing of Revenue Recognition Transferred at a point in time where CDW is principal 1,743.4 364.0 1,939.3 443.8 4,490.5 Transferred at a point in time where CDW is agent 111.0 19.6 79.6 15.5 225.7 Transferred over time where CDW is principal 59.1 2.6 82.0 47.8 191.5 Total Net sales $ 1,913.5 $ 386.2 $ 2,100.9 $ 507.1 $ 4,907.7 (1) Net sales by geography is generally based on the ship-to address with the exception of certain services that may be performed at, or on behalf of, multiple locations. Such service arrangements are categorized based on the bill-to address. (2) Includes items such as delivery charges to customers. Three Months Ended September 30, 2018 (in millions) Corporate Small Business Public Other Total Geography (1) United States $ 1,704.1 $ 340.0 $ 1,875.1 $ 6.8 $ 3,926.0 Rest of World 2.4   444.8 447.2 Total Net sales 1,706.5 340.0 1,875.1 451.6 4,373.2 Major Product and Services Hardware 1,379.5 284.0 1,530.8 363.6 3,557.9 Software 227.5 44.3 291.0 45.6 608.4 Services 83.1 6.6 48.8 40.5 179.0 Other (2) 16.4 5.1 4.5 1.9 27.9 Total Net sales 1,706.5 340.0 1,875.1 451.6 4,373.2 Sales by Channel Corporate 1,706.5    1,706.5 Small Business  340.0   340.0 Government   639.3  639.3 Education   793.1  793.1 Healthcare   442.7  442.7 Other    451.6 451.6 Total Net sales 1,706.5 340.0 1,875.1 451.6 4,373.2 Timing of Revenue Recognition Transferred at a point in time where CDW is principal 1,571.6 321.0 1,764.6 402.8 4,060.0 Transferred at a point in time where CDW is agent 85.7 16.9 64.3 12.3 179.2 Transferred over time where CDW is principal 49.2 2.1 46.2 36.5 134.0 Total Net sales $ 1,706.5 $ 340.0 $ 1,875.1 $ 451.6 $ 4,373.2 (1) Net sales by geography is generally based on the ship-to address with the exception of certain services that may be performed at, or on behalf of, multiple locations. Such service arrangements are categorized based on the bill-to address. (2) Includes items such as delivery charges to customers. Nine Months Ended September 30, 2019 (in millions) Corporate Small Business Public Other Total Geography (1) United States $ 5,525.5 $ 1,119.2 $ 5,271.7 $ 24.8 $ 11,941.2 Rest of World 8.1   1,546.2 1,554.3 Total Net sales 5,533.6 1,119.2 5,271.7 1,571.0 13,495.5 Major Product and Services Hardware 4,411.6 937.5 4,297.6 1,143.0 10,789.7 Software 784.8 145.0 808.0 213.3 1,951.1 Services 284.0 20.8 150.8 206.3 661.9 Other (2) 53.2 15.9 15.3 8.4 92.8 Total Net sales 5,533.6 1,119.2 5,271.7 1,571.0 13,495.5 Sales by Channel Corporate 5,533.6    5,533.6 Small Business  1,119.2   1,119.2 Government   1,860.2  1,860.2 Education   1,981.0  1,981.0 Healthcare   1,430.5  1,430.5 Other    1,571.0 1,571.0 Total Net sales 5,533.6 1,119.2 5,271.7 1,571.0 13,495.5 Timing of Revenue Recognition Transferred at a point in time where CDW is principal 5,033.4 1,054.7 4,908.0 1,383.7 12,379.8 Transferred at a point in time where CDW is agent 333.7 59.4 190.9 42.0 626.0 Transferred over time where CDW is principal 166.5 5.1 172.8 145.3 489.7 Total Net sales $ 5,533.6 $ 1,119.2 $ 5,271.7 $ 1,571.0 $ 13,495.5 (1) Net sales by geography is generally based on the ship-to address with the exception of certain services that may be performed at, or on behalf of, multiple locations. Such service arrangements are categorized based on the bill-to address. (2) Includes items such as delivery charges to customers. Nine Months Ended September 30, 2018 (in millions) Corporate Small Business Public Other Total Geography (1) United States $ 4,999.9 $ 997.1 $ 4,740.5 $ 24.0 $ 10,761.5 Rest of World 6.2   1,398.0 1,404.2 Total Net sales 5,006.1 997.1 4,740.5 1,422.0 12,165.7 Major Product and Services Hardware 4,013.4 829.4 3,876.7 1,136.9 9,856.4 Software 699.6 131.2 730.5 154.0 1,715.3 Services 242.3 20.6 120.5 124.8 508.2 Other (2) 50.8 15.9 12.8 6.3 85.8 Total Net sales 5,006.1 997.1 4,740.5 1,422.0 12,165.7 Sales by Channel Corporate 5,006.1    5,006.1 Small Business  997.1   997.1 Government   1,551.3  1,551.3 Education   1,902.4  1,902.4 Healthcare   1,286.8  1,286.8 Other    1,422.0 1,422.0 Total Net sales 5,006.1 997.1 4,740.5 1,422.0 12,165.7 Timing of Revenue Recognition Transferred at a point in time where CDW is principal 4,581.3 940.0 4,445.8 1,277.4 11,244.5 Transferred at a point in time where CDW is agent 279.9 50.5 160.7 37.1 528.2 Transferred over time where CDW is principal 144.9 6.6 134.0 107.5 393.0 Total Net sales $ 5,006.1 $ 997.1 $ 4,740.5 $ 1,422.0 $ 12,165.7 (1) Net sales by geography is generally based on the ship-to address with the exception of certain services that may be performed at, or on behalf of, multiple locations. Such service arrangements are categorized based on the bill-to address. (2) Includes items such as delivery charges to customers. The following table presents Net sales by major category for the three and nine months ended September 30, 2019 and 2018 . Categories are based upon internal classifications. Three Months Ended September 30, 2019 2018 Dollars in Millions Percentage of Total Net Sales Dollars in Millions Percentage of Total Net Sales Notebooks/Mobile Devices $ 1,325.6 27.0 % $ 1,124.8 25.7 % Netcomm Products 567.3 11.6 582.4 13.3 Desktops 448.4 9.1 349.0 8.0 Video 353.3 7.2 320.5 7.3 Enterprise and Data Storage (Including Drives) 283.6 5.8 297.4 6.8 Other Hardware 946.0 19.3 883.8 20.3 Total Hardware 3,924.2 80.0 3,557.9 81.4 Software (1) 707.2 14.3 608.4 13.9 Services (1) 243.8 5.0 179.0 4.1 Other (2) 32.5 0.7 27.9 0.6 Total Net sales $ 4,907.7 100.0 % $ 4,373.2 100.0 % Nine Months Ended September 30, 2019 2018 Dollars in Millions Percentage of Total Net Sales Dollars in Millions Percentage of Total Net Sales Notebooks/Mobile Devices $ 3,537.5 26.2 % $ 3,069.4 25.2 % Netcomm Products 1,641.9 12.2 1,568.5 12.9 Desktops 1,183.6 8.8 1,013.2 8.3 Video 972.7 7.2 904.4 7.4 Enterprise and Data Storage (Including Drives) 849.2 6.3 816.2 6.7 Other Hardware 2,604.8 19.3 2,484.7 20.5 Total Hardware 10,789.7 80.0 9,856.4 81.0 Software (1) 1,951.1 14.4 1,715.3 14.1 Services (1) 661.9 4.9 508.2 4.2 Other (2) 92.8 0.7 85.8 0.7 Total Net sales $ 13,495.5 100.0 % $ 12,165.7 100.0 % (1) Certain software and services revenues are recorded on a net basis for accounting purposes. As a result, the category percentage of net revenues is not representative of the category percentage of gross profits. (2) Includes items such as delivery charges to customers.