Most database combine searches using Boolean Logic:

The most common are:

AND - search terms joined with AND will find results that include all the search terms, but not one of the search terms on its own. This narrows your search.

OR - search terms joined with OR will find results were one of more of the search terms are featured. This broadens your search.

NOT - search terms joined with NOT excludes terms so that each search results does not contain any of the terms that follow it. This narrows your search. NOT is not usually recommended in a systematic review as it may exclude relevant information depending on the content.

                      AND                                                           OR                                                         NOT

PROXIMITY (or ADJACENCY) - Finds words near to each other or in the same sentence.

This table summaries search techniques in the various database along with other search functionality including truncation, plurals, phrase searching and wildcards.

• Database Comparison Table

What is a search filter?

Search filters also known as Hedges are a set of search terms or a search strategy to identify a topic or aspect e.g. a study type (Randomised Control Trials-RCTs) or an age group (children or the elderly). These are tried and tested strategies intended for repeated use and are routinely combined with specific search terms (e.g. RCT filter AND breast cancer terms AND tamoxifen terms). Search filters are designed for specific databases and should not be applied to other databases. They will also have different focuses with some having a focus on sensitivity and others on precision. 

In the Medline Search example you will find an RCT filter applied to a search.

Humans filter - For most databases it is not enough to use the human limit function because not all papers about humans have been indexed as such. It is better not to limit to humans at all except where it is built into the database search filter, e.g. the Ovid filter uses a double negative to eliminate animal studies from your search results therefore by default limiting to human. See the example in the Medline search

Filters should be used with caution not all filters are equally well designed or fit for purpose. It might be preferable to screen out studies rather than applying a filter to ensure no relevant studies are missed.

 Below are a few questions that you would want to ask before using a filter:

• Is this filter well designed?
• Is it fit for purpose?
• Has it been designed for the exact database I'm using?
• Will it be effective and suitable for my context?
• Are any relevant studies being missed by applying the filter?

How do you find filters?

• Cochrane Library Handbook lists ln section 4.4.7 lists filters
• Canada Drug Agency lists filters by Database
• CEBMAs - Centre for Evidence Based Management also has a list of filters
• ISSG - InterTASC Information Specialists' Sub-Group - Search filter resource
• McMaster Knowledge Health Refinery (HKR) - Hedges Project.

Remember when applying a filter each line needs to be entered separately into the database. You can also save a filter as a saved search and then apply it and combine it with other searches as needed. The filter always has to be the right filter for that specific database otherwise it will not work.