Table:
|  |  | Three Months Ended |  |
| --- | --- | --- | --- |
|  | December 31, 2019 | September 29, 2019 | December 31, 2018 |
|  |  | (Unaudited, in millions) |  |
| Operating income | $460 | $336 | $443 |
| As percentage of net revenues | 16.7% | 13.1% | 16.8% |
Paragraphs:
Operating income in the fourth quarter grew on a sequential and year-over-year basis to $460 million compared to $336 million and $443 million in the prior and year-ago quarters, respectively.
Operating income
Question:
What is the average percentage of net revenues for the period December 31, 2019 and 2018?