For longer papers, containing a wealth of information, writing an appendix is a useful way of including information that would otherwise clutter up the paper and mire the reader in over-elaborate details.
Whilst often referred to as a 'dump,' the information should be relevant to the paper, and must be laid out with the same care and attention.
The key point to remember, when you are writing the appendix, is that the information is non-essential. If it were removed, the paper would still be perfectly understandable, and it is simply a place for extra information.
Many papers include the appendix for the peer reviewing process, and then remove it before publication, allowing the quality of the raw information to be verified.
It is usually good practice to include your raw data within the appendix, laying it out in a neat table and allowing anybody to recheck your results.
The tables that you include within the body of the paper will then be concise and uncluttered, allowing the reader to pick out the important information. Any tables and figures included in the appendix should be numbered as a separate sequence from the main paper, often as Fig A1, Fig A2 etc.
When writing an appendix, it used to be traditional to include the working and any explanations for statistical calculations within the appendix, although this is becoming less common in the age of spreadsheets.
It is a good idea to include a little explanation of what computer program you used, including the version, as each individual version may have its own interpretation. You can also indicate why you used it, as well as additional information that may be useful, such as how many decimal places you rounded to.
The appendix is a good place to put maps, extra photographs and diagrams of apparatus, if you feel that it will help the reader to understand, remembering that the body of the paper should be understandable without them.
For chemistry-based papers, where long and complicated names for compounds are common, you can include the full name in the appendix, and refer to them in the paper by the accepted abbreviation.
The answer to that really is as long as it needs to be. The appendix should be streamlined, and not too loaded with information, but there is a lot of flexibility.
The appendix does not count towards the word count for your research paper, so you can set the length to suit. If you have a very long and complex paper, with an extremely long appendix, it is a good idea to break it down into sections, allowing the reader to find relevant information quickly.
Whilst writing an appendix should not affect the quality or final mark for your research paper, a well-formatted and informative appendix can create a good impression. This attention to detail is what makes your paper stand out from the rest.