It is helpful to create a diagram of the important concepts for your topic as shown below.
Question: How effective is a day care service for individuals with dementia or alzheimer's disease in alleviating caregiver burden?
Notice that non-essential words such as "service" and "individuals" and "alleviating" have been removed from the strategy. You could also leave out the word "effective" unless the search results set is extremely large. For each additional term that you add to your search with the Boolean operator AND, the results will become smaller.
This search could be entered in PubMed as:
day care AND (dementia OR alzheimer's) AND caregiver burden
About Boolean Operators
The Boolean operators AND, OR, and NOT are used to combine search terms in databases.
Combining Concepts with "AND"
Example: dementia AND day care
The Boolean operator AND will retrieve citations that contain both concepts "day care" and "dementia" in the same record (note the dark grey segment). As a result, you will retrieve fewer records.
Combining Concepts with "OR"
Example: dementia OR alzheimer's
The Boolean operator OR will retrieve citations that contain either "dementia" or "alzheimer's" in the records (as shown by the entire shaded areas of both circles). As a result, you will retrieve a larger number of records.
Combining Concepts with "NOT"
Example: dementia NOT alzheimer's
The Boolean operator NOT will exclude citations containing the second term (as shown by the shaded area of the first circle.) Use NOT cautiously because you may exclude some records that could be relevant to your search.