Level 1. These are reviews of a number of studies and are an even higher standard than RCTs. This can involve a pooling of the data from a number of studies (a meta analysis) or a critique / analysis / summary of multiple comparable studies to draw a conclusion or recommendation.
Level 2. "Randomized Clinical Trials" or RCTs are the gold standard of research. Such an approach randomizes people into two or more groups for comparison of outcomes. Randomization serves to minimize biased or unpredictable effects on the outcome.
Level 3. This kind of study has a matched control group for comparison but participants are not randomized into the comparison groups.
Level 4. Research based on following a group of individuals before and after an intervention or service. Each person as their own baseline and control but there is no formal control (ie, comparison) group.
Level 5. Single case reports, anecdotes, observations, and series of self reports. This is how scientific inquiry begins with observations and subsequent inquiries.