When I teach writing, I always ask my students, “How many of you hate outlines?” usually more than half of the students raise their hands, and well they should.
Most of us were taught to use outlines in fifth grade or so. We had an assignment and were required to turn in an outline with all the Roman numerals and capital letters and then Arabic numerals and lower-case letters—and if you got down to a fifth level of complexity, Greek letters. The teacher read your outline and gave it a checkmark and handed it back to you, with the assumption (stated or unstated) that your writing would follow that outline literally to the letter.
I point this out to illustrate the underlying assumption that our teachers unwittingly taught us: that it is possible to know what you are going to write before you write it. Depending on what kind of a writer you are, this assumption could appear to be anywhere from obvious to ridiculous. And you would be right.
If you are writing about something very concrete, say, the process of turning on a computer or making an omelet, an outline will serve you well. It does not matter how you say things or whether you tell readers to “Push the on button” or “turn the computer on,” to “crack the egg” or “de-shell the egg,” they are likely to understand and be able to comply. But when you start to write about more abstract or complex ideas, then the words you choose to describe idea A might change the content of idea B, and the more that happens, the further you can get from the ideas mapped out in your outline. Then you have a choice: do you follow the new ideas or follow the outline?
There is no single easy answer to this question. One way to decide is by considering your rhetorical situation. Does your audience expect or demand that you follow an agreed-upon path? Do you know enough about your topic to go in one or the other direction, or will you need to do more research? What will be most useful or interesting for you, the writer?
In the professional academic world, this happens all the time. Journals and conferences ask for a 250 word abstract of a 10-20 page essay you haven’t written yet, and no one admits that an abstract like this is a complete work of fiction, with the writer pretending to be able to predict the future. But that’s what you are being asked to do.
Having said this, I must also point out that outlines can be life-savers, especially when you have either very little or too much time to write. When you have very little time, a short outline will ensure that you cover all the needed points. In college, when we had “blue book” exams and had to answer, say, three essay questions in two hours, I would number the parts of each question and pace myself so that I covered all of them. When you have too much time, in contrast, the same thing applies. A student writing a research paper over the course of a semester is going to have lots of ideas and notes about different research sources as the months pass. An outline is a place to record those ideas so that they don’t disappear.
The point in both cases is that the outline is a tool primarily for the use of the writer. It is a working document written, metaphorically, in pencil not with a chisel on stone. It can and should change as your ideas change. Most of my outlines lately are written in two colors of ink on a piece of scrap paper or a cocktail napkin.