Table:
|  |  | Year Ended December 31, |  |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2017 |
|  |  | (dollars in thousands) |  |
| Cost of revenue | $149,215 | $144,349 | $142,867 |
| Gross profit | $427,308 | $393,542 | $339,118 |
| Gross margin | 74.1% | 73.2% | 70.4% |
Paragraphs:
Cost of Revenue, Gross Profit, and Gross Margin
Cost of revenue increased $4.9 million, or 3%, in 2019 as compared to 2018. The increase in cost of revenue was primarily due to $12.1 million in increased personnel expenses, stock-based compensation, and overhead costs, $3.9 million in increased content costs, $3.6 million in increased amortization of acquired intangible assets, and $1.6 million in increased capitalized software amortization. These increased costs were partially offset by $16.4 million in decreased external implementation professional service costs. These costs were incurred to service our existing customers and support our continued growth. The improvement in gross margin was primarily due to a higher mix of subscription revenue, which carries a higher gross margin.
Cost of revenue increased $1.5 million, or 1%, in 2018 as compared to 2017. The increase in cost of revenue was primarily due to $6.0 million in increased capitalized software amortization and $4.5 million in increased content costs. These increased costs were partially offset by $6.6 million in decreased amortization of acquired intangible assets and $2.9 million in external implementation service costs. These costs were incurred to service our existing customers and support our continued growth. The improvement in gross margin was primarily due to a higher mix of subscription revenue, which carries a higher gross margin.
Question:
Why did gross margin improve between 2018 and 2019?
Answer: The improvement in gross margin was primarily due to a higher mix of subscription revenue, which carries a higher gross margin