As you begin searching through the literature for evidence, you will come across different types of publications. Below are examples of the most common types and explanations of what they are. Although systematic reviews and meta-analysis are considered the highest quality of evidence, not every topic will have an SR or MA.
Remember, a literature review provides an overview of a topic. There may or may not be a method for how studies are collected or interpreted. Lit reviews aren't always obviously labeled "literature review"; they may be embedded within sections such as the introduction or background. You can figure this out by reading the article.
Systematic reviews explore all research related to a given question. Reviews are gathered using a specific, defined set of criteria. Reviewed and state whether or not they answer the research question.
Meta-analysis is a study that combines data from OTHER studies. All the studies are combined to argue whether a clinical intervention is statistically significant by combining the results from the other studies. For example, you want to examine a specific headache intervention without running a clinical trial. You can look at other articles that discuss your clinical intervention, combine all the participants from those articles, and run a statistical analysis to test if your results are significant. Guess what? There's a lot of math.