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Characteristics of included studies, by researching regions
| Eastern zone18–50 | Central zone51–56 | Western zone57–68 | National wide69–72 | |
|---|---|---|---|---|
| Number of studies | 33 | 6 | 12 | 4 |
| Published year | ||||
| 1990–2000 | 9 | 1 | 5 | 0 |
| 2001–2005 | 15 | 4 | 3 | 4 |
| 2006–2012 | 9 | 1 | 4 | 0 |
| Sample size | ||||
| <500 | 8 | 2 | 7 | 0 |
| 500–1499 | 15 | 3 | 3 | 0 |
| 1500–2999 | 8 | 1 | 1 | 0 |
| ≥3000 | 2 | 0 | 1 | 4 |
