style - Does DRY (Don't Repeat Yourself) Apply to Documentation?

In programming, it's usually accepted that DRY code is better code in most situations.

Does this principle also apply to documentation?

I'm asking about the documentation output, not necessarily the source material (there are tools for keeping the source content DRY even if there's a lot of repeated content).

Example: suppose I have the following content:

• Some task
  
  
  
  
  • Prerequisite setup task
  
  
  • How to do X
  
  
  • How to do Y
  
  
  
  

Should "How to do X" and "How to do Y" explicitly say "Before doing this, make sure to do Prerequisite setup task"? What are some good rules to follow when deciding if documentation should repeat itself?

I intended this question to be about end-user documentation. In some context, such as code or compliance documentation, the answer may drastically change. See Chenmunka's answer for some of those implications.

Answer

No. Don't repeat yourself is a good content management rule, which is what it is in programming as well. If you have two instances of the same thing it becomes harder to manage them. If it were not for the management issues it raises, there would be no point to the DRY doctrine.

But the same pieces of information is often needed in more than one place. Programming handles this by a function call. Functions exist so that a piece of code that performs a useful function can be used in many places. That code is, in fact, repeated, it is simply repeated at run time, rather than write time.

The equivalent of run time for documentation is read time. Just as the same function needs to be repeated many times at runtime in software, the same information needs to be repeated many times at read time in documentation.

This is why some documentation tools have elaborate content reuse systems: to enable content that occurs once at write time to occur many times at read time.

The reason that the same information needs to occur many times across a documentation set (or multiple documentation sets, if you have more than one product or version) is that people do not read the whole documentation set at once (or ever), they dip into it opportunistically when they need something, particularly when they are stuck on some operation. They are only going to read a small portion of the documentation set at any given time. For them, the topic or page that they find is the documentation set. (I call this, Every Page is Page One.) That page needs to have the information they need to complete their task. It can link to related material where needed, but there is still going to be a lot of repetition necessary to create usable docs.