Search filters are:

• also known as hedges
• pre-cooked search strings that are designed to limit search results
• designed to be highly-sensitive, precise, or a balance of both

Commonly, search filters limit search results by:

• age group
• publication type
• study design

They are sometimes available at the click of a button in library databases, but don't let that deceive you! A lot of work goes into developing an effective search strategy for filters, and they often include various combinations of subject headings, textwords, and wildcards.

PubMed - Randomized Controlled Trials 

From the Cochrane Handbook, Section 6.4.11, Box 6.4.a:

(((((((((randomized controlled trial [pt]) OR controlled clinical trial [pt]) OR randomized [tiab]) OR placebo [tiab]) OR drug therapy [sh]) OR randomly [tiab]) OR trial [tiab]) OR groups [tiab])) NOT ((animals [mh] NOT humans [mh])) 

From Glanville, J. M., Lefebvre, C., Miles, J. N. V., & Camosso-Stefinovic, J. (2006). How to identify randomized controlled trials in MEDLINE: ten years on. Journal of the Medical Library Association, 94(2), 130–136.:

(((((((clinical trial[pt]) OR randomized[tiab]) OR placebo[tiab]) OR clinical trials[mh]) OR randomly[tiab]) OR trial[ti])) NOT ((animals[mh]) NOT (animals[mh] AND humans[mh])) 

OVID Medline - Randomized Controlled Trials

From the Cochrane Handbook, Section 6.4.11, Box 6.4.a:

This work is openly licensed via CC BY-NC-SA 4.0. For information on this guide contact Erica Nekolaichuk, Faculty Liaison & Instruction Librarian at the Gerstein Science Information Centre.