SG&A expenses, which decreased year-over-year, primarily consist of third-party marketing costs, professional fees and compensation and benefits and occupancy and other costs of our corporate and executive offices, which includes group-wide finance and accounting, treasury, legal and governance, human resource oversight and similar costs. 

In 2020, salary and service costs, which tend to fluctuate with changes in revenue, decreased $1,399.4 million, or 12.8%, compared to 2019. Salary and related service costs in 2020 decreased $644.3 million, or 9.3%, year-over-year, primarily as a result of the severance and furlough actions we took in the second quarter of 2020. Also, during 2020, we reduced salary and service costs by $162.6 million related to reimbursements and tax credits under government programs in several countries (see Note 1 to the consolidated financial statements). Third-party service costs, which include expenses incurred with third-party vendors primarily when we act as a principal when performing services for our clients, decreased $755.1 million, or 18.5%, year-over-year reflecting the decrease in revenue and the impact of actions we took to align our cost structure. Occupancy and other costs, which are less directly linked to changes in revenue than salary and service costs, decreased $83.3 million, or 6.8%, in 2020 as compared to 2019. Operating profit decreased $523.5 million to $1,598.8 million. Operating margin decreased to 12.1% from 14.2%, and EBITA margin decreased to 12.8% from 14.8% year-over-year. In 2020, operating profit, operating margin and EBITA margin included a net decrease aggregating $171.1 million related to the COVID-19 repositioning costs recorded in the second quarter and asset impairment charges recorded in the fourth quarter, partially offset by an increase related to reimbursements and tax credits under government programs in several countries (see Note 1 to the consolidated financial statements). 

In 2020, net interest expense increased $5.5 million year-over-year to $189.5 million. Interest expense on debt decreased $27.6 million to $199.6 million, primarily reflecting a reduction in interest expense from our refinancing activity at lower interest rates in the second half of 2019 and the first quarter of 2020, partially offset by a loss of $7.7 million on the early redemption of the remaining $600 million principal amount of the 2020 Notes in the first quarter of 2020 and the interest expense from the issuance of the 4.20% Notes in April 2020 (see Note 7 to the consolidated financial statements). Interest income in 2020 decreased $28.0 million year-over-year to $32.3 million primarily due to lower rates. 

Our effective tax rate for 2020 increased year-over-year to 27.1% from 26.0%. The non-deductibility in certain jurisdictions of a portion of the COVID-19 repositioning costs recorded in the second quarter of 2020 had the effect of increasing our effective tax rate for 2020. This increase was substantially offset by a lower effective tax rate on our foreign earnings resulting from a change in legislation. In 2019, income tax expense was reduced by $10.8 million primarily from the net favorable settlements of uncertain tax positions in certain jurisdictions. After considering these items, our effective rate for 2020 would have approximated the rate for 2019. 

Net income - Omnicom Group Inc. in 2020 decreased $393.7 million to $945.4 million from $1,339.1 million in 2019. The year-over-year decrease is due to the factors described above. Diluted net income per share - Omnicom Group Inc. decreased to $4.37 in 2020, compared to $6.06 in 2019, due to the factors described above, as well as the impact of the reduction in our weighted average common shares outstanding resulting from repurchases of our common stock through March 2020, net of shares issued for restricted stock awards, stock option exercises and the employee stock purchase plan during the year. As discussed above and in Note 1 to the consolidated financial statements, our results of operations for 2020 included COVID-19 repositioning costs and asset impairment charges, which were partially offset by an increase attributable to reimbursements and tax credits under government programs in several countries. The after-tax impact of these items on net income - Omnicom Group Inc. was $146.8 million. 

## CRITICAL ACCOUNTING POLICIES 

The following summary of our critical accounting policies provides a better understanding of our financial statements and the related discussion in this MD&A. We believe that the following policies may involve a higher degree of judgment and complexity in their application than most of our accounting policies and represent the critical accounting policies used in the preparation of our financial statements. Readers are encouraged to consider this summary together with our financial statements and the related notes, including Note 2, for a more complete understanding of the critical accounting policies discussed below. 

## Estimates 

We prepare our financial statements in conformity with U.S. GAAP and are required to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. We use a fair value approach in testing goodwill for impairment and when evaluating our equity method investments to determine if an other-than-temporary impairment has occurred. Actual results could differ from those estimates and assumptions. 

## Acquisitions and Goodwill 

The evaluation of potential acquisitions is based on various factors, including specialized know-how, reputation, geographic coverage, competitive position and service offerings of the target businesses, as well as our experience and judgment. 

Our acquisition strategy is focused on acquiring the expertise of an assembled workforce in order to continue to build upon the core capabilities of our various strategic business platforms and agency brands through the expansion of their geographic 

