In order to be thorough and systematic, multiple databases must be used and grey literature must also be searched if it is applicable. The Cochrane Collaboration recommends searching a minimum of three databases:
The Cochrane Collaboration also specifies that:
Your inclusion/exclusion criteria are extremely important and allow you to make your results more precise after you've conducted a search to find as much as you can that touches on your topic of interest.
Just as in clinical or bench research, a detailed methodology of your search strategy, results and inclusion/exclusion criteria must be included in your review so that it is reproducible. PRISMA standards require you to fill out a flow diagram and provides a helpful checklist to determine if your SR is complete.
Remember the evidence pyramid and how few systematic reviews and meta-anaylses there are? This can be attributed to a few factors:
The Cochrane Collaboration estimates about a year to complete the process, and this is often by teams of people working together on one review.