Table 1.
Review of methodological strengths and weaknesses of included studies
| Study design (see key) | N | Appropriate definition of retention | Multi-site (appropriate estimation of retention) | Non-specific sample (generalizability of findings) | Appropriate data analyses | Broad range of variables measured/no missing data | All additional contextual factors accounted for | |
|---|---|---|---|---|---|---|---|---|
| Developed countries | ||||||||
| Adams [22] | R | 561 | + | − | − | + | − | − |
| Althoff [14] | P | 867 | + | + | − | + | + | − |
| Blank [19] | P | 921 | + | − | − | − | + | ? |
| Dombrowski [23] | P | 247 | − | − | + | + | + | ? |
| Giordano [15] | R | 2619 | + | + | − | + | + | ? |
| Horberg [20] | R | >23,000 | + | + | + | − | − | ? |
| Kelly [26] | P | 168 | + | − | + | + | + | ? |
| Lourenço [16] | R | 7621 | + | + | + | + | − | ? |
| Mcmahon [18] | R | 4966 | − | − | + | − | − | ? |
| Noysk [12] | R | 6152 | + | − | + | + | − | ? |
| Rebeiro [17] | P | 61,438 | + | + | + | + | + | ? |
| Richey [21] | C | 99 | + | − | + | − | − | ? |
| Scahfer [24] | P | 251 | − | − | + | + | + | ? |
| Tedaldi [7] | P | 1441 | + | + | + | + | − | ? |
| Tobias [10] | P | 1000 | + | + | + | + | + | ? |
| Waldrop-Valverde [25] | P | 210 | + | − | − | + | + | ? |
| Developing countries | ||||||||
| Allam [37] | P | 1690 | − | − | − | + | + | ? |
| Boyles [27] | P | 1803 | + | − | + | + | − | ? |
| Charurat [28] | P | 5760 | + | + | + | + | − | ? |
| Janssen [29] | R | 223 | + | − | + | + | − | ? |
| Koole [30] | R | 4147 | + | + | + | + | − | ? |
| Krumme [40] | P | 610 | + | − | + | + | − | − |
| Mekuria [31] | R | 836 | + | + | + | + | − | ? |
| Mutasa-Apollo [32] | R | 3919 | + | + | + | + | − | ? |
| Palombi [38] | R | 3749 | + | + | + | + | − | ? |
| Thida [33] | R | 5718 | + | + | + | + | − | ? |
| Ugoji [34] | R | 5176 | + | + | + | + | − | ? |
| Vella [35] | R | 2835 | + | + | + | + | − | ? |
| Vuylsteke [39] | R | 414 | + | + | − | + | − | + |
| Yang [36] | R | 822 | + | − | + | + | − | ? |
R retrospective study, P prospective study, C cross-sectional study, + identified in the article, – not identified in the article, ? not addressed in the article