(dollars in millions)  Increase/ (Decrease)
(dollars in millions)  Increase/ (Decrease)
(dollars in millions)  Increase/ (Decrease)
(dollars in millions)  Increase/ (Decrease)
Years Ended December 31,
2019
2018
2019 vs. 2018
2019 vs. 2018
Cost of services 
$ 31,772
$ 32,185
$ (413)
(1.3)%
Cost of wireless equipment 
22,954
23,323
(369)
(1.6)
Selling, general and administrative expense
29,896
31,083
(1,187)
(3.8)
Depreciation and amortization expense 
16,682
17,403
(721)
(4.1)
Media goodwill impairment 
186
4,591
(4,405)
(95.9)
Consolidated Operating Expenses
$101,490
$ 108,585
$ (7,095)
(6.5)
consolidated operating expenses .
operating expenses for our segments are discussed separately below under the heading “ segment results of operations .
” .
cost of services cost of services includes the following costs directly attributable to a service : salaries and wages benefits materials and supplies content costs contracted services network access and transport costs customer provisioning costs computer systems support and costs to support our outsourcing contracts and technical facilities .
aggregate customer care costs which include billing and service provisioning are allocated between cost of services and selling general and administrative expense .
cost of services decreased $ 413 million or 1 .
3 % during 2019 compared to 2018 primarily due to decreases in network access costs a product realignment charge in 2018 -see “ special items ” decreases in employee-related costs resulting from the voluntary separation program and decreases in digital content costs .
these decreases were partially offset by increases in rent expense as a result of adding capacity to the networks to support demand and the adoption of the new lease accounting standard in 2019 regulatory fees and costs related to the device protection package offered to our wireless retail postpaid customers .
cost of wireless equipment cost of wireless equipment decreased $ 369 million or 1 .
6 % during 2019 compared to 2018 primarily as a result of declines in the number of wireless devices sold as a result of an elongation of the handset upgrade cycle partially offset by a shift to higher priced devices in the mix of wireless devices sold .
selling general and administrative expense selling general and administrative expense includes salaries and wages and benefits not directly attributable to a service or product bad debt charges taxes other than income taxes advertising and sales commission costs call center and information technology costs regulatory fees professional service fees and rent and utilities for administrative space .
also included is a portion of the aggregate customer care costs as discussed above in “ cost of services .
” .
selling general and administrative expense decreased $ 1 .
2 billion or 3 .
8 % during 2019 compared to 2018 primarily due to decreases in employee-related costs primarily due to the voluntary separation program a decrease in severance pension and benefits charges -see “ special items ” .
the acquisition and integration related charges in 2018 primarily related to the acquisition of yahoo ’ s operating business -see “ special items ” and a net gain from dispositions of assets and businesses in 2019 -see “ special items ” partially offset by increases in advertising expenses sales commission and bad debt expense .
the increase in sales commission expense during 2019 compared to 2018 was primarily due to a lower net deferral of commission costs as a result of the adoption of topic 606 on january 1 2018 using a modified retrospective approach .
depreciation and amortization expense depreciation and amortization expense decreased $ 721 million or 4 .
1 % during 2019 compared to 2018 primarily due to the change in the mix of net depreciable assets .
media goodwill impairment the goodwill impairment charges recorded in 2019 and 2018 for verizon media were a result of the company ’ s annual impairment test performed in the fourth quarter -see “ critical accounting estimates ” .
media goodwill impairment the goodwill impairment charges recorded in 2019 and 2018 for verizon media were a result of the company ’ s annual impairment test performed in the fourth quarter -see “ critical accounting estimates ” .

Question: What caused the decrease in the cost of services?
Answer: decreases in network access costs, a product realignment charge in 2018 (see “Special Items”), decreases in employee-related costs resulting from the Voluntary Separation Program and decreases in digital content costs