Sources of data and yield of searches in systematic reviews. Types of unpublished data are in rows, and sources of unpublished data are in columns
| Type of unpublished data | National and international trial registries (n=85) | Subject-specific trial registries (n=4) | Pharmaceutical industry trials registries (n=16) | Conference proceedings (n=71) | Contacting individuals or organisations (n=84) |
| Unpublished completed studies | 55 | 4 | 16 | 46 | 53 |
| Grey literature only | 22 | 0 | 0 | 25 | 21 |
| Ongoing studies only | 6 | 0 | 0 | 0 | 2 |
| Unreported data from published studies only | 2 | 0 | 0 | 0 | 8 |
| Searches that yielded results (%) | 75 (88) | 3 (75) | 15 (94) | 63 (89) | 76 (90) |