Multi-word search terms are searched as phrases in Ovid. There is no need to place quotation marks around these phrases unless the phrase contains operators for Ovid (e.g. “and”, “or”, “not”, “use” etc.) or special characters (e.g. à, ç, è, é, ê etc.).
In PubMed Advanced Search you have to place quotes around the multiword terms to do the same. But in that case, wildcards cannot be used and some phrases cannot be searched in PubMed, even though they are present in one or more records.
Using the PubMed entry page search box, many but not all phrases are recognized and searched using its MeSH Translation Table. Phrases in PubMed are not recognized when hyphens or quotation marks, a Medical Subject Heading (MeSH), or truncation are used.
In addition, Ovid has an adjacency operator:
ADJn is a positional operator. It is very useful to search for specific phrasing, complex or expert searching. When this is used in between 2 search terms, you will search for both terms, in any order, with up to (n-1) words in between.
The number of “n” can be from 1 to 99, but in most searches a number from 1 to 10 will be fine.
You can test different numbers for n (for example 3, 4, 5) and look at the extra records of these subsequent searches. Decide from these results which “n” you prefer.
Be aware that ADJn is important in free text searching!
For example, the search (kidney adj3 diseases).ti,ab. retrieves records that contain the words kidney and diseases with up to 2 words in between, in any order, in titles and abstracts.
The ADJn operator is more specific than the AND operator (kidney AND diseases), but less specific than a phrase (kidney diseases). With kidney ADJ3 diseases you will retrieve records containing kidney diseases, kidney and liver diseases, diseases of the kidney.
The ADJn can also be used with truncated terms, for example (kidney* ADJn disease*).ti,ab.
PubMed does not perform adjacency searching. However many phrases are recognized by the MeSH Translation Table and used in PubMed’s Automatic Term Mapping (ATM).