A truly excellent post on content reuse guidelines and thresholds was posted today to the stc-single-sourcing mailing list.
Julie Kumasaka asked: "Does anyone use a guideline or has anyone come across a guideline recommending what percentage (or some other measure) of text from two sources should be the same to be worth single-sourcing them?"
Mark Baker (email@example.com) posted an excellent reply:
I see this topic has generated a lot of discussion, which, unfortunately, I don't have time to study properly at the moment, so I will offer the following thought and hope it is not totally redundant with what has already been said.
The point is not to reduce redundancy in content. The point is to reduce redundancy in effort. Reducing redundancy in content is just a means to that end, and it should only be done when it can be shown to achieve that end. In the case of conditional text, it makes a huge difference whether the conditional content is volatile or stable. Setting up and testing conditional text is more work than simply writing the content twice. The win comes if using conditional text for some content lets you reduce the effort of maintaining other content. If the stable parts of your content are conditionalized, you only have to go through the pain of setting up the conditions once. You set it and forget it, and then just update the unconditional content whenever it changes.
In this case, using conditional text saves you a lot of effort, no matter the proportion of conditional to unconditional text. On the other hand, if the volatile parts of the content are the conditional ones, you will constantly be maintaining the conditions, still writing two different pieces of content, and having additional work to test your conditions. In this case using conditional text will not save you effort and may create additional effort. Again, this is true no matter what percentage of the content is conditional.
In short, if you conditionalize stable elements of your content, you win. If you conditionalize volatile elements of you content, you break even at best, and probably lose. Looking at the balance of volatile vs. stable content is therefore a better indicator than looking at the percentage of conditional content.