Apostrophes are punctuation marks used to indicate possession.  The placement of an apostrophe depends upon the word you are making possessive.

1. Singular possessive. Make a singular noun possessive by adding an apostrophe and an s.

   Example: Long's theory (2003) suggested...

   Note that singular nouns ending in s still take an apostrophe and an additional s (i.e., Jones's theory).
2. Plural possessive (regular). Make a plural noun possessive by adding an apostrophe.

   Example: The Nurses' Association published a code of ethics...

3. Plural possessive (irregular). Make an irregular plural noun (such as women or children) possessive by adding an apostrophe.

   Example: The Akron Children's Hospital.

There are some common apostrophe errors that are easy to avoid. Do not use an apostrophe:

1. When making a noun plural.

   Incorrect: The studies' of Jones and Barnum...
   Correct: The studies of Jones and Barnum... or Jones's and Barnum's studies...

2. When making an acronym or a number plural.

   Incorrect: CEO's of major corporations in the 1950's...
   Correct: CEOs of major corporations in the 1950s...

3. When constructing contractions. Apostrophes can also be use to create contractions (words like can't and didn't); however, contractions are rarely used in academic writing.

   Incorrect: Walden University doesn't recognize Ground Hog Day as an official holiday.
   Correct: Walden University does not recognize Ground Hog Day as an official holiday.