| Auteurs | Country | Duration (year) | Number of VVF |
| Our study | Guinea | 10 | 1770 |
| S. M. Gueye [5] | Senegal | 6 | 123 |
| Benchekroun [6] | Morocco | 30 | 1050 |
| R. Kimassoum [7] | Chad | 3 | 363 |
| Bernis L [8] | Developing countries | - | - |