People doing systematic reviews often use filters to only include randomized controlled trials which are widely seen as the best kind of research for evidence-based medicine. However, search filters exist for many other types of study design including Observational Studies and Diagnostic Studies.
The Cochrane Library Handbook lists filters for RCTs in the Medline database on the Ovid and PubMed platforms. You can find these filters in the Cochrane Handbook (Part 2, section 6.4.11)
Filter by study design e.g. only RCTs
Other pre-tested search filters for different types of experimental design and for different databases which you can copy and paste line by line into your search are also available from:
You can save search filters after you have typed them in by choosing to save your search history – then you can simply re-run them when you wish to apply them to a specific search. The filters look complicated but you should be able to cut and paste them into the search box in the database. Remember though – each line of the filter is a separate search on the database and you will need to combine them in exactly the same way as set out in the filter before using AND to apply them to your search strategy.