An addendum is another name for an appendix, which may be a necessary component of a research paper in APA style, if the writer wishes to include extra information that does not fit in the paper. Some examples of extra information include surveys used in research, mathematical proofs and definitions of key terms.
A paper may have more than one appendix, if necessary. If there is only one appendix, no title is needed. Multiple appendices should be titled using subsequent letters of the alphabet; for example, "Appendix A", "Appendix B", "Appendix C," but without quotation marks. Refer to appendices by their assigned labels within the body of the paper. There is no specific format for an appendix because there are different types of information that could be presented in an appendix. However, writers should conform to the general rules of APA style when designing an appendix. Appendices should appear after reference lists and tables.
General APA Guidelines
Reports written in APA style should be double-spaced with one inch margins. The recommended font is 12-point Times New Roman. A running head, or page header, should appear at the top of every page with the page number flush right and the title of the paper flush left, written in all capital letters. The running head should not be more than 50 characters. All tables should be numbered and titled and should appear after the reference list and before additional appendices.