All words in a Basic search are automatically connected by the OR operator. This means that a Basic search query consisting of the phrase "chronic asthma" will return all articles related to either the word "chronic" or the word "asthma", with articles related to both words ranking higher in the list of results. This behavior can lead to an extremely large number of matching articles, depending on the terms in your query.

To obtain more specific results, you may wish to use an Advanced search. The Advanced search interface allows you to specify up to four separate query phrases. These phrases can be connected using the boolean AND / OR / BUTNOT operators to obtain greater searching precision.

For field selections other than "All Fields", the words in a given query phrase are connected with the AND operator. Note that a single query phrase using the "All Fields" selection behaves exactly like the Basic search.

Here are some additional tips to help you refine your search:

1. If your search is unsuccessful, you may need to remove some "noise" words from the query. This interface automatically removes words such as "a", "and", "the", etc. that add nothing to the specificity of the search, but you may also wish to trim from your query any other terms that do not add significant precision.
   
   
2. All terms that begin with a given string can be searched on by appending "*" to the end of the term.
   
   
3. For searches specifying the Journal field, you must use the accepted PubMed Journal Abbreviations. Consult the PubMed Journal Browser to find the proper abbreviation.
   
   
4. Author names must be in the format: LastName Initial(s). The initials can be omitted.
   
   
5. You may also want to restrict your search based on publication date. Searches performed using a relative date restriction will return all articles published within the specified number of days from current date. Searches using an absolute date restriction return only articles that were published between a specified minimum and maximum date.