To err is human, and that includes when we search databases. And while there is always a possibility of typos and errors in searches, the chance of errors increases with the complexity and length of search strategy used.

Most databases, including Pubmed, have mechanisms of alerting a searcher of certain errors found within a search by providing “warnings”, but unfortunately not all typos cause a functional error, and in those instances the database will not provide a warning and the user may end up with unintended search results.

Functional Errors in PubMed

Functional errors in PubMed trigger a warning, as they impede in the function of the database to conduct the search as written.

Quoted phrase not found in phrase index

Not all phrases (string of words you enclose in double quotes) can be found in PubMed due to how PubMed indexes phrases. 

“progenitor cell transplantation”

The easiest solution to this error is remove the double quotes, however this can lead to unintended results if you are not careful. There are several things to keep in mind if you simply remove the quotation marks. 

• If you remove double quotes from a phrase not found, but are using a specific field code, the search would be broadened by implying that there are Boolean operator AND in between each word, but the field code prevents PubMed from automatically mapping.  

“progenitor cell transplantation”[tiab] → progenitor cell transplantation[tiab]

• If you remove double quotes from a phrase not found, but are not using any field codes at the end of your phrase, the automatic translation by PubMed would become much broader than intended as it will add additional mapping to MeSH terms and word variations for each separate term.

“progenitor cell transplantation” → progenitor AND cell AND transplantation

PubMed recommends using proximity searching to fix this error. Proximity searching is a newer feature in PubMed that allows the user to control how close terms are to one another. In the example below it would only retrieve results in which all 3 terms were found within 3 words of one another.

“progenitor cell transplantation”[tiab:~3]

The last and most extreme solution to this functional error is to switch to a database that does not restrict phrase searching, such as Embase.

The asterisk in your search was ignored

If you are using an asterisk as a wildcard (truncation) in a search strategy, you must use 4 or more characters.

The easiest way to fix this error is simply lengthen the root word to at least 4 characters to truncate and include all possible endings.

The following term(s) were ignored:

This error is usually caused by a typo where something in your search is unbalanced or unpaired, including parentheses, quotation marks, and duplicate boolean operators. 

If you are unable to quickly locate where the issue is, go to Advanced Search and click on the ! under details. This will expand out your entire search strategy and highlight where the error is located.

Common Search Typos

Since these are typos, they often do not trigger a warning within PubMed so it’s important to carefully check your search strategy to make sure everything is correct.

Boolean Operators

Boolean operators (AND, OR, NOT) must be fully capitalized. If they are not capitalized or only the first letter is capitalized, the search translates it as a term and not a Boolean operator, meaning Or would find the word Or in the record but would not OR together two terms. 

If a Boolean operator is omitted PubMed will automatically insert the AND operator. Since AND and OR produce significantly different results, an unintended AND where the search needs an OR would cause a serious alteration in the results, but since it is a legitimate search technique, there would be no warning from PubMed.

The Similar articles feature is one of many built-in tools that PubMed offers for finding references. This tool works by using a specific article as a starting point, and using it’s own internal algorithm identifies articles that are similar to the original article within PubMed. The similar articles tool is especially helpful if you cannot find a sufficient number of relevant references by conducting a regular search in PubMed.

Similar articles to a specific article can be located by scrolling down the record (it’s found directly underneath the abstract and conflict of interest statement), or you jump directly to the section of the page for Similar articles found in Page Navigation on the right-hand column of the record.

Under the Similar articles heading you will see several references listed and you will find the rest of references when clicking See all similar articles beneath. The number of references found this way typically is manageable, not overly large.

As was mentioned above, there is an underlying algorithm that determines what Similar Articles search finds, i.e. determines the criteria for the relevance of these references to the one you’ve used to find them.

It is possible that what the algorithm determined was important to focus on from the initial article to retrieve similar citations is not the part of reason you selected the article as relevant. You may see a mix of references that match the topic of your initial reference very closely and the ones that are less relevant but still related. The default sorting order of this search results is always Best Match, with most relevant (as determined by the search algorithm) on the top but you may find very relevant references down the list of search results as well.

Just as you would in a traditional PubMed search, within the Similar Articles portion you can adjust the results by changing the sorting order or using the filters found on the left-hand pane – with some caution. You can also click on Advanced under the search box and use the Search Builder and History and Search Details to combine your Similar articles search (expressed as Similar articles for PMID number of an original article) with other searches or build on it.

NOTE: it is most important to remember is that in most cases, for best results, Similar Articles search should be complementary to a regular PubMed search and not the main method of searching.