| Differentiation of revenue and cost of revenue |
Revenue and cost of revenue by source are as follows (in thousands): | | | | | | | | | | | | | | | | | | Three months ended September 30, | | Nine months ended September 30, | | 2015 | | 2014 | | 2015 | | 2014 | Revenue: | | | | | | | | Products | $ | 76,555 |
| | $ | 92,708 |
| | $ | 217,237 |
| | $ | 257,415 |
| Rentals | 7,574 |
| | 16,966 |
| | 28,146 |
| | 45,954 |
| Services | 3,813 |
| | 7,087 |
| | 11,963 |
| | 21,284 |
| | $ | 87,942 |
| | $ | 116,761 |
| | $ | 257,346 |
| | $ | 324,653 |
| Cost of revenue: | | | | | | | | Products | $ | 49,520 |
| | $ | 57,315 |
| | $ | 145,387 |
| | $ | 155,048 |
| Rentals | 3,387 |
| | 8,272 |
| | 13,315 |
| | 22,444 |
| Services | 2,160 |
| | 3,074 |
| | 6,283 |
| | 9,042 |
| Depreciation | 1,648 |
| | 2,022 |
| | 5,355 |
| | 6,051 |
| | $ | 56,715 |
| | $ | 70,683 |
| | $ | 170,340 |
| | $ | 192,585 |
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