Search mode errors occur because the descriptors are not searched in the [mesh] field, either explicitly or through automatic mapping; because they are not exploded (and, thus, their more specific terms are not retrieved) when searching in the text words [tw] field; because automatic mapping is disabled by truncating descriptors or enclosing MeSH phrases in double quotes; or because terms are not searched in free-text fields such as the title and abstract.
Note that searching all fields [all] is equivalent to automatic mapping, unless the terms are truncated or enclosed in double quotes, because they turn off automatic mapping.
* Truncating a term or phrase enclosed in double quotes has no effect.
The procedure for performing phrase searches in PubMed includes the following steps: (a) truncating the final term, (b) joining terms with a hyphen, (c) enclosing the phrase in double quotes, and (d) using a field tag.
Meanwhile, it should be taken into account that MeSH terms searched in the [tw] field are not exploded and do not retrieve more specific terms (lower recall), while terms searched in the [all] field are processed automatically unless truncated or enclosed in double quotes, in which case they are searched in all fields.
[43] (phrase final term to enclosed in search as a double quotes) phrase OS "decision support system" 2,731 CS decision support system * 10,394 Truncating Truncate Eddens L, et al.