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Search FAQ

How does the search work?

NLM's search engine is a concept-based information retrieval system. The search engine matches search terms to concepts within NLM's web site. Your search results will include documents with exact word matches as well as documents with concept matches. A concept-based search enables you to retrieve information on topics important to you even if you do not enter the term.

For example, look for information on lung diseases caused by certain kinds of work, you might type in the search terms “occupational lung diseases.” Although there may be no exact keyword match, the search engine retrieves information on the concept of “occupational lung diseases,“ which includes articles containing information on occupational asthma, occupational cancer, and specific types of lung hazards such as solvents.

How results are displayed: In the search results, the most relevant categories under a folder are displayed first, based on the relevance score for all the documents found within that category. Under the categories, the results are displayed in order of relevancy of the documents found.

Results are ranked by the number of matching words or concepts in the document compared to the number of words on the document. For example, a 50-word document with two matching terms is ranked higher in the results than a 250-word document with two matching terms.

Controlled Vocabulary: Searches are expanded using a thesaurus. The thesaurus contains a list of all the words found in all documents on the site and additional synonyms from NLM's MeSH (Medical Subject Headings). When there is a match between a search term and a word in the controlled vocabulary, the controlled vocabulary word is added to the search as an “or” term.

Stemming: Stemming is performed on search terms. Stemming removes suffixes such as “-ing,” “-ed,” “-en,” and “s.” For example, a search on “coughing” would retrieve any results that include the verb stem “cough.” A search on “legs” would also retrieve articles containing the term “leg.” Stemming is not performed on search terms entered in all capitals, such as AIDS.

Case of Search Terms: The search engine matches words and concepts regardless of case. For example, a search on “National Library” will also retrieve results containing “national library.”

International Spellings: The search engine treats foreign language terms with accent symbols in the standard normalized fashion as follows:

For example, a Spanish query of niño can be typed as “nino” and will retrieve documents that include the term “niño” or “nino.”

Stop words: The search engine ignores words that occur very frequently in documents. These words include:

Spelling: The search engine checks words against terms collected from the web site. For words that appear to be misspelled, the user is prompted to select an alternative spelling from a list that contains alternative spellings found on the site.


The documents retrieved don't have all the words I searched for. How can I make sure all my words are in the documents?

The search engine looks for any of the words you typed in.

To force the search engine to retrieve only documents containing all the words requested, use the "+" symbol. For multiple words, you must use put a + sign in front of every word you want to be in every document. (e.g. +lipitor +cholesterol)


Can I search phrases?

The search engine does not allow phrase searching. You can require that all the words be in your document.

To force the search engine to retrieve the word, use the “+” symbol. For multiple words, you must use put a + sign in front of every word you want to be in every document. (e.g. +lipitor +cholesterol)


Is Boolean searching allowed?

These Boolean operators are available to you: AND, OR, NOT, *, +

AND Use "AND" when you want both concepts to appear in all results. This search uses the conceptual model, so the documents retrieved may not have the exact word in the search.
OR Use “OR” when you want either concept to appear in the results. This search uses the conceptual model, so the documents retrieved may not have the exact word in the search.
NOT Use “NOT” when you do not want a particular concept to appear in the results.
* Use “*” as a wildcard to find all related words to a particular prefix (e.g “neo*” would find neoplasms, neoplastic, etc). The conceptual model is not used in this search, so only variations of the exact word searched will be found.
+ Use “+” when you want the exact word to appear in the results. For multiple words, you must use put a + sign in front of every word you want to match. (e.g. +lipitor +cholesterol). The conceptual model is not used in this search, so stemmed variants of the exact word searched will be found.

What types of files are searched?

The search engine finds these file types:


I got a message that says the word wasn't found with an option to replace the word with another word. What is this?

The search engine couldn't find the word the way you spelled it. It looks at all the words it “knows” and offers you a choice of words to replace it.

So, for example, if you type in “fibermyalgia,” the search engine returns a page saying it couldn't find that word but shows you “fibromyalgia,” the correct spelling.

Clicking on the correct spelling and then on the “Change search” button to run the search with the correct spelling.


What do I do if I get a message saying no results found for the search?

There are a few reasons why you could have received this message.

Your word is misspelled, but the search engine can't find a substitute for it. For example, if you had typed in “dyabeets” when you meant “diabetes,” the search engine won't be able to recognize it.

The kind of information you are looking for isn't available from the NLM web site. If you are looking for the title of a book or journal articles, you won't find them listed here. Or, if you are looking for information on the health of Neanderthal people, you will not find this type of information on

But if you search PubMed, NLM's bibliographic database of journal articles (, you will find many articles with the term Neanderthal in them.

Or if you search LocatorPlus, NLM's online catalog (, you will find books on the subject.


What do the folders mean?

The folders on the left side of the screen divide the results of your search into five categories:

National Library of Medicine

Contains documents from the main NLM web site ( Results in this folder are created from information on the library's hours, directions to the library, news about the library, interlibrary loan, searching the library's online catalog, the Visible Human project, searching the library's databases, and many other of the library's programs and services.

Health Information - MedlinePlus

Contains documents from MedlinePlus, the National Library of Medicine's consumer health web site ( You will find results in this folder if you search for information on diseases or conditions such as diabetes, high blood pressure, fibromyalgia, breast cancer, preventing heart disease, or other health concern.

Profiles in Science

Contains documents from archival collections of prominent twentieth-century biomedical scientists ( The collections contain published and unpublished materials, including books, journal volumes, pamphlets, diaries, letters, manuscripts, photographs, audiotapes, and other audiovisual materials. You will find results in this folder if you searched for information on Linus Pauling, Barbara McClintock, or other prominent scientists.


Contains online exhibitions from NLM's History of Medicine Division and other online audiovisual programs ( You would find results in this folder if you had searched for information on the Frankenstein exhibit, the Breath of Life, art objects at NLM, or other online exhibits.


Contains documents stored in the National Library of Medicine Archives. The NLM archives contain older Web pages from the National Library of Medicine’s main Website. All pages in the archives are considered to be of permanent value. They are moved from the current site to the archives when their contents are no longer current. NLM has made a commitment to keep them available permanently. For more information see About the NLM Archives.


How do I see everything in a folder?

The initial results page displays up to 20 items from the National Library of Medicine folder and up to 5 results from each of the other folders. To see everything in a folder, you may:


Can I search in one folder?

The only folder that has a separate search utility is MedlinePlus. To search MedlinePlus, go to


What does the infinity symbol. next to a search result listing mean?

The symbol. next to a search results is an infinity symbol, and appears for all documents that NLM has assigned permanence ratings and other metadata. Click on the infinity symbol to view the permanence information for that document. Permanence information includes: title, URL, date last updated, date first published, permanence level, and a link to all of the metadata for the document. Click "Permanence level" to see the definitions of NLM's four permanence levels. NLM uses permanence levels to indicate which NLM Web documents can change, and which will remain permanently available. For more information, please see Developing Permanence Levels and the Archives for NLM’s Permanent Web Documents.

What if Help doesn't answer my question?

If this Help does not answer your questions, contact Customer Service and we will do our best to help you.


Last reviewed: 16 June 2008
Last updated: 04 October 2004
First published: 19 November 2002
Metadata| Permanence level: Permanence Not Guaranteed