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I'm updating a technical document, and the phrase "left as an exercise for the reader" is used twice. However, that phrase is something that just irks me, so I'm thinking about removing it and revising those sentences.

One of the instances is this quotation:

...a precise account of the principles and practices is left as an exercise for the reader.

So I have two questions:

  1. Is this phrase one that can be considered appropriate for technical writing?
  2. Is there a better way to phrase this idea in a technical document? If so, what is it?
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up vote 17 down vote accepted

The only time that might be appropriate is where the document is part of some course literature. Finding the related information could be considered as part of the learning process.

If the document is for use in the 'real' world, e.g. describing the function of a piece of equipment, I too would be annoyed to find this. If part of the description is to be found elsewhere, then either the relevant parts of the reference material should be inserted into this document (in-line, or as a footnote, or as an appendix), or a reference (document name/number/section or link, if on-line) should be quoted.

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It depends on the material in question. I would allow it only in:

  • Academic literature when a student is expected to work through explicit exercises; and

  • Professional literature when the “exercise” is straightforward for a professional.

Otherwise, “left as an exercise for the reader” is like “beyond the scope of this document”—fine if you have an accompanying reference, but otherwise superfluous and a bit self-subversive.

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I like "beyond the scope of this document" myself. – Russell Borogove Mar 13 '12 at 18:31
Along with "beyond the scope of this document", you might also consider providing a reference where the reader can learn more about the topic being elided. – Ryan Thompson Mar 13 '12 at 19:40
Sure, "beyond the scope" is sometimes used as a cop-out for failing to explain something adequately. But I think it's legitimate as a way to say that you are not going to incorporate the entire contents of some other document here, but you don't want to just leave the reader hanging, wondering where the rest of the explanation is or why he doesn't get it. Like if you're writing instructions on how to, say, use an accounting program, and you get to a point where you say the user can incorporate an MS Word document, it's fair to say, "Use of MS Word is beyond the scope of this document." – Jay Mar 13 '12 at 21:06
@RussellBorogove: Yeah, I don’t have a problem with it entirely. I meant to refer only to its use without a reference to somewhere the topic is in scope. After all, anything not mentioned in your document is, by definition, beyond. – Jon Purdy Mar 13 '12 at 21:11

This phrase comes from textbooks, where it is meant literally. The author presents some information, then presents a problem or question involving the information just taught, works through a partial solution, generally of the most difficult part, and then rather than take up a lot of space with details of calculations or a rehash of material discussed in previous chapters, says that the remainder of the problem is "left as an exercise for the reader". That is, the reader should be able to work out the remainder of the problem himself, and probably should do so to be sure he understands the material.

The phrase is sometimes used as a joke. When a writer gets to the difficult part of a problem and he doesn't want to take the trouble to solve it, or doesn't know how to solve, he says this is "left as an exercise for the reader", as if it was too trivial to waste space in the book discussing, when really the issue is that he can't figure out the answer himself. Like, "When you go hiking, bring a bottle of some tranquilizer, like Valium, with you. Then if you are encounter a grizzly bear or other dangerous wild animal in the forest, simply give him two of these tranquilizers to calm him down and you can safely pass. How to get the bear to take the tranquilizers is left as an exercise for the reader." Like any joke, whoever first used it was a clever comedian, but it gets tedious when overused.

The literal usage is inappropriate in a technical book that is not presented as a textbook with exercises. As a joke, whether it is appropriate depends on the overall tone of the book.

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This usage is largely considered bordering on tongue in cheek. It's a little informal for technical documentation, but the author has deemed its playfulness to be a worthy means to lighten the discourse. "Beyond the scope" is more formal, though not completely equivalent. "Left as an exercise" indicates the topic is worthy, but best done by oneself; "outside scope" may imply that it isn't actually germane to the point of the matter at hand. This phrase is largely accepted by the mathematics (and hence computer science) community.

Incidentally it does not necessarily come from textbooks; it is common in technical articles in mathematics.

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I find I don't quite agree with previous answers. The phrase "left as an exercise for the reader" means to me that the author has covered the basics and any competent reader should be able to fill in the details. I don't see a problem with it if the author's claim is correct -- that is, has he actually covered the basics?

The quoted instance has something that would make me suspicious about the claim:

...a precise account of the principles and practices is left as an exercise for the reader.

Leaving practices for the reader to work out is one thing, but principles? That sounds as if it goes beyond just filling in details.

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