| continent | Authors | country | Frequency | Year |
|
| Farsi I F. [10] | Senegal | 32% | 1997 |
|
| Dicko B. [9] | Mali | 30% | 2001 |
|
| ILesanmi O A. [15] | Nigeria | 15% | 1996 |
|
| Our study | Mali | 30% | 2009 |
| Europe | Denis A. [16] | France | 47.9% | 1990 |
|
| Saunders N J. | England | 90% | 1996 |
|
| Koo M R. | Netherlands | 44.4% | 1998 |
| USA | Laros | USA | 82% | 1995 |
|
| Schiff E. | USA | 61.4% | 1996 |