Table:
|  |  | Years Ended December 31, |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
|  |  | (in thousands) |  |
| Revenue: |  |  |  |
| Royalty and license fees | $198,124 | $219,708 | $167,923 |
| Total revenue | 198,124 | 219,708 | 167,923 |
| Operating expenses: |  |  |  |
| Cost of revenues | 8,460 | 13,291 | 6,308 |
| Research, development and other related costs | 83,613 | 78,892 | 75,809 |
| Litigation | 1,656 | — | 288 |
| Amortization | 88,075 | 88,544 | 90,340 |
| Total operating expenses (1) | 181,804 | 180,727 | 172,745 |
| Total operating income (loss) | $16,320 | $38,981 | $(4,822) |
Paragraphs:
Product Licensing Segment
(1) Excludes operating expenses which are not allocated on a segment basis.
Product Licensing revenue for the year ended December 31, 2019 was $198.1 million as compared to $219.7 million for the year ended December 31, 2018, a decrease of $21.6 million. The decrease in revenue was primarily due to the timing and duration of minimum guarantee contracts up for renewal and executed, decreased NRE services revenue, as well as a decrease in per-unit royalty revenue in 2019 as compared to 2018.
Question:
Why was Product Licensing revenue decreased in 2019 compared to 2018?
Answer: the timing and duration of minimum guarantee contracts up for renewal and executed, decreased NRE services revenue, as well as a decrease in per-unit royalty revenue