Observational study - a study that draws inferences from a population or sample where the independent variable cannot be controlled due to ethical concerns or logistical constraints. For example, observational research may be done because participants and controls cannot be randomly assigned.
Example, the Souza et al. study (1) is observational because knee pain cannot be randomly assigned. This study compared individuals with knee pain to "matched-controls" without knee pain.
- Souza, R. B. & Powers, C. M. (2009) An evaluation of hip strength and femoral structure in women with and without patellofemoral pain. The American Journal of Sports Medicine, 37(3), 579-587. doi: 10.1177/0363546508326711