(Unaudited, in thousands)
| Three Months Ended December 31, | Year Ended December 31, | |||||||||||||||
| 2020 | 2019 | 2020 | 2019 | |||||||||||||
| Gross profit | $ | 128,874 | $ | 91,936 | $ | 465,954 | $ | 346,325 | ||||||||
| Gross margin | 55.3 | % | 55.1 | % | 55.2 | % | 55.2 | % | ||||||||
| Adjustments to reconcile gross profit to non-GAAP gross profit: | ||||||||||||||||
| Stock-based compensation expense | 686 | 574 | 2,592 | 2,409 | ||||||||||||
| Deferred compensation plan expense | 281 | 29 | 931 | 54 | ||||||||||||
| Amortization of acquisition-related intangible assets | - | - | - | 110 | ||||||||||||
| Non-GAAP gross profit | $ | 129,841 | $ | 92,539 | $ | 469,477 | $ | 348,898 | ||||||||
| Non-GAAP gross margin | 55.7 | % | 55.5 | % | 55.6 | % | 55.6 | % | ||||||||
