Time element and control groups in the study designs of the articles surveyed. The survey of the time element was not considered relevant for studies having only a single possible study design (pure epidemiology, epidemiology–other, controlled drug trials, drug use–other, and genetic association). The study types clinical descriptive, pure epidemiology, epidemiology–other, controlled drug trials, drug use–other, and animal studies were not considered eligible for the survey of control groups and were excluded from this analysis.
| Time Element | % (n) | Control Groups | % (n) | Use of Disease Control Groups | % (n) |
|---|---|---|---|---|---|
| Cross-sectional | 83.0 (151) | No control groups | 9.3 (16) | Clinical hypothesis testing | 52.6 (10) |
| Prospective longitudinal | 7.1 (13) | Healthy controls | 82.0 (141) | Mainly laboratory | 57.4 (55) |
| Retrospective longitudinal | 9.9 (18) | Disease controls | 41.9 (72) | Genetic association | 12.8 (6) |
| Intradisease controls | 11.0 (19) | ||||
| Family-based controls | 1.7 (3) |