These might include such items as
• Technical descriptions of (unusual) statistical procedures
• Detailed tables or computer output
• Figures that were not central to the arguments presented in the body of the report
• Computer code used to obtain results.
In all cases, and especially in the case of computer code, it is a good idea to add some text sentences as comments or annotations, to make it easier for the uninitiated reader to follow what you are doing.
It is often difficult to find the right balance between what to put in the appendix and what to put in the body of the paper. Generally you should put just enough in the body to make the point, and refer the reader to specific sections or page numbers in the appendix for additional graphs, tables and other details.