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| 2002–2006 | 2007–2011 | ||||
| Research Design | No. | (%) | No. | (%) | p-value |
| Prospective | 67 | 74 | 62 | 63 | 0.66 |
| Retrospective | 23 | 25 | 33 | 34 | 0.14 |
| Case control | 1 | 1 | 3 | 3 | - |
| Prospective | 1 | - | 0 | - | - |
| Retrospective | 0 | - | 3 | - | - |
| Total | 91 | 98 | |||
| Sample sizes | |||||
| up to 200 | 22 | 24 | 15 | 15 | 0.5 |
| 201 to 500 | 21 | 23 | 25 | 26 | |
| 501 to 1,000 | 16 | 18 | 20 | 20 | |
| >1,000 | 32 | 35 | 38 | 39 | |
