Table:
|  |  | Year Ended |  |  | Change |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- |
| (Dollars in thousands) | 2019 | 2018 | 2017 | 2019 over 2018 |  | 2018 over 2017 |  |
| Research and development | $96,611 | $84,472 | $88,573 | $12,139 | 14% | $(4,101) | (5)% |
| % of net sales . | 3.2% | 3.8% | 3.0% |  |  |  |  |
Paragraphs:
Research and development
Research and development expense consists primarily of salaries and other personnel-related costs; the cost of products, materials, and outside services used in our R&D activities; and depreciation and amortization expense associated with R&D specific facilities and equipment. We maintain a number of programs and activities to improve our technology and processes in order to enhance the performance and reduce the costs of our solar modules.
The following table shows research and development expense for the years ended December 31, 2019, 2018, and 2017:
Research and development expense in 2019 increased compared to 2018 primarily due to increased material and module testing costs and higher employee compensation expense.
Question:
What are components of research and development expense?
Answer: Research and development expense consists primarily of salaries and other personnel-related costs; the cost of products, materials, and outside services used in our R&D activities; and depreciation and amortization expense associated with R&D specific facilities and equipment.