Secondary studies synthesise the results from primary studies:

• Systematic reviews have a defined set criteria for finding and including primary studies for critical analysis. Methodology from these primary studies is often evaluated and results synthesised.
• Meta-analysis is a statistical procedure for combining quantitate data from studies evaluating the same outcomes (e.g. data from a study of a large population showing a strong correlation and data from a smaller study finding no effect may be weighted and combined to find a small overall effect). Meta-analyses are often done as an extension of systematic reviews.
• Clinical practice guidelines are often issued by governing medical bodies and are based on the best available research evidence (normally this is meta-analyses and systematic reviews).