Table:
|  |  |  | Year Ended December 31, |  |  |
| --- | --- | --- | --- | --- | --- |
|  | 2019 | % Change | 2018 | % Change | 2017 |
|  |  |  | (in thousands, except percentage data) |  |  |
| Net revenue | $287,372 | (0.1)% | $287,756 | 6.2% | $270,908 |
| Percentage of net revenue | 28.8% |  | 27.2% |  | 26.1% |
| Contribution income | $67,282 | (4.1)% | $70,142 | 9.8% | $63,865 |
| Contribution margin | 23.4% |  | 24.4% |  | 23.6% |
Paragraphs:
2019 vs 2018
SMB segment net revenue was flat for the year ended December 31, 2019 compared to the prior year, primarily due to a decline in net revenue of our
network storage products, substantially offset by growth in net revenue of our switch products. Geographically, net revenue grew in APAC, but declined in Americas and EMEA.
Contribution income decreased for the year ended December 31, 2019 compared to the prior year, primarily as a result of lower gross margin attainment, partially offset by lower operating expenses as a proportion of net revenue. Contribution margin decreased for the year ended December 31, 2019 compared to the prior year, primarily lower gross margin attainment mainly resulting from foreign exchange headwinds due to the strengthening of the U.S. dollar as well as higher provisions for sales returns.
2018 vs 2017
SMB segment net revenue increased for the year ended December 31, 2018 compared to the prior year, primarily due to growth in switches, partially offset by the decrease in network storage. SMB experienced growth in net revenue across all regions. SMB net revenue was further benefited by lower provisions for sales returns deemed to be a reduction of net revenue.
Contribution income increased for the year ended December 31, 2018 compared to the prior year, primarily due to increasing net revenue and improved gross margin performance not being met with proportionate increases in operating expense compared to the prior period.
Question:
What accounts for the decrease in contribution income in 2019?
Answer: Lower gross margin attainment, partially offset by lower operating expenses as a proportion of net revenue.