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| High‐income | Upper middle‐income | Lower middle‐income | Low‐income |
| Arnoff 1984 | Haffejee 1988 | Bryan 1985 | Nathan 2005 |
| Barson 1985 | Odio 1986 | Filali 1993 | Sharma 1996 |
| Congeni 1984 | Tuncer 1988 | Girgis 1987 | |
| Del Rio 1983 | Rodriguz 1985 | ||
| Jacobs 1985 | Girgis 1988 | ||
| Narciso 1983 | |||
| Peltola 1989 | |||
| Steele 1983 | |||
| Wells 1984 |
