Number of studies per World Bank (2021) income level
| Income level | Number of studies | % of total studies |
| High income | 24 | 42.9 |
| Upper middle income | 9 | 16.1 |
| Lower middle income | 13 | 23.2 |
| Low income | 8 | 14.3 |
| Multiple* | 2 | 3.6 |
| Total | 56 | 100.0 |
*Both studies categorised under multiple were across countries classified as high income and upper middle income.