Systematic reviews constitute the most prodigious contribution that scientific-practitioners can make to a given body of clinical knowledge. When conducted in a rigorous and objective fashion, the pooled treatment effects yielded from this design are considered the highest level of applied clinical evidence that exists. It is much more of an academic/empirical task versus applied experimental and observational designs. Yet, the time and experience needed to conduct a systematic review often impede these pursuits by researchers. (However, they are greatly needed and should be undertaken if at all possible! I'm going to start my first one soon.)
True experiments such as randomized controlled trials are not feasible for most researchers due to lack of funding, logistical support, and available resources. Also, researchers should first show observational evidence of a treatment effect before conducting a randomized controlled trial.
Prospective cohort studies can generate important measures of incidence and relative risk, as well as longitudinal data. However, this type of design means you are moving forward in time and are dependent upon enough observations being generated from you methodology to have adequate statistical power. The logistics and time associated with this design also tend to hinder its application in busy clinical environments.
The next highest level of evidence is the retrospective cohort design and it is easily applied in a busy clinical environment. This is a retrospective design so the data already exists. One defines a cohort with inclusion and exclusion criteria. Then, members of the cohort are separated into independent groups according to some sort of exposure or non-exposure to a treatment, intervention, or risk. They are then followed up to a certain point in time to see if they did or did not have the outcome. There are obvious selection and observation biases associated with this design but it yields important measures of relative risk and many years of data can be mined for longitudinal or large-scale cohort analyses. Survival analyses are perfect for this type of design when establishing the 1-year, 3-year, or 5-year survival or "time-to-event" rates of an outcome. They are also relatively inexpensive to conduct and time-friendly. This research design is much more preferable to case-control designs.