Though there are no specific instructions for an addendum in APA style, there are instructions for adding an appendix or multiple appendices to APA formatted reports. Additionally, the APA style guide includes instructions for citing an addendum or appendix from a source on a reference page. Regardless of the reason for adding an addendum, the general guidelines for APA style must be observed.
Purpose
APA style is commonly used for psychology and other social sciences, which use a mix of qualitative and quantitative research. These reports can often include extra information that may be important for understanding the study, but appear distracting when placed in the body of the essay.
For example, a data set for qualitative research may include examples of survey questions or definitions for research-specific vocabulary.
Quantitative research data sets may include mathematical proofs. These types of extra, but important, information can be included as appendices at the end of a paper.
Formatting
An appendix should appear after the reference page and tables, at the end of an APA formatted paper. Each appendix must appear on a separate page and include a title, centered at the top of the page. If there is only one appendix, the title should be "Appendix." If more than one appendix is required, add letters in ascending order as needed; for example, "Appendix A," "Appendix B" and "Appendix C." Use these titles when referring to the information within the appendix in the body of the paper. The first paragraph of the appendix should appear flush left, but the rest should be indented.
Tables
There is a specific APA format for styling tables, which should appear after the reference list and before any appendices. Tables should only be used for large amounts of data; any small amounts of data should simply be presented in the text. Like appendices, tables should be titled, using numbers instead of letters.
For example, "Table 1," "Table 2" and "Table 3." Refer to tables in the text using these labels.
Tables also require brief, specific titles after the label, for example,
"Table 1: Number of Respondents."
Citing an Appendix or Addendum
Another way to add an addendum or appendix is when it is used as a source in a paper. In this case, the following information is needed: author, title of work, location of publication and publisher. According to Walden University, cite the appendix according to the following format: Author. (year). Appendix A of Title of work. Location: Publisher. This must be done as a part of your reference list.