Database content constantly changes and records are added retrospectively so years of coverage are fluid.
It is important to understand that the years indicated in a database name – such as Ovid MEDLINE(R) ALL <1946 to present> often refer to the date the catalogue started (1946 in this case) and not to the date of the earliest publications indexed in the database, which in Medline go back to the late 1700s.
For many bibliographic databases it is not possible to accurately define the years of coverage. That is why we keep these records for systematic reviews:
If you select 'limit to humans' you risk losing all non-indexed and pre-print publications in a database which have not been indexed as 'human'. This could be thousands of records.
A safer alternative is to search for the things we don't want (in this case animal studies) and remove them from the results. Adjust the line numbers below as appropriate.
6. [last line of search - your line number will be different]
7. exp animals/ not humans/
8. 6 not 7
Some databases, such as Ovid MEDLINE(R) ALL, include many thousands of non-indexed records and pre-print publications. These new records flood in from electronic publications and there is a time lag before they can be fully indexed or described. The records received from publishers are basic and don't (yet) include subject headings. Of course these non-indexed records include the latest research, so we don't want to lose them.
There are the only two limits we recommend: