The Publication Manual of the American Psychological Association (6th edition) stresses the importance of properly citing information from outside sources to acknowledge contributions to your paper, avoid plagiarism and allow readers to check your information. Citing a single author who wrote multiple works requires differentiating the sources by the date or by the addition of a letter with the date for works written in the same year.
References Page Format
When determining the order of entries on the references list when one author has multiple works, place the oldest publication first. For sources written by the same person in the same year, start with the work with the title that comes first alphabetically -- ignoring articles like "The" and "An" -- and include a letter after the date. Start with "a," followed by "b" in the next reference and so on, like these examples: Stafford, J. (2012a). About the boy. Developmental Psychology, 12, 300-312. Stafford, J. (2012b). Staying the Course. Journal of American Psychology, 22, 24-32.
In-Text Citation Format
In APA style, the author and date given in a signal phrase or parenthetical citation tell the reader which source the information comes from. When you have an author with multiple works written in different years, the date indicates which source the reference reflects. If the author wrote multiple publications in the same year, add letters to the end of each year to distinguish the works from each other, mirroring the citation found on the references list. Such a citation looks like this:
According to Stafford (2012b), dramatic changes resulted from the study.
Since this parenthetical citation includes the letter "b" after the date, the reader knows to look for the second source on the references list written by Stafford in 2012.