Is Sentence Length a True Indicator of Readability?

Have you ever been reading something and reached the end of a sentence, then had to start reading the sentence again because you forgot what it was about?  Do you think it was simply because it was a long sentence? Or were there other reasons?

We are often told that short sentences are more readable than long ones. However, this is an oversimplification. There are many factors that determine how “readable” a sentence is, and quite a few of them depend more on the reader than on the writer. I contend that sentence length, by itself, is at best a poor and inconsistent measure of readability.

What is “Readability”?

For purposes of this article, readability refers to how well a piece of writing (which I define as a passage that’s at least a paragraph in length) conveys its message, or conversely, how well the reader comprehends the message.

The factors that influence readability can be classified as either external (dependent on the reader) or internal (dependent on the writer). External factors include:

  • The reader’s reading comprehension skill
  • The reader’s vocabulary
  • The reader’s knowledge of and interest in the subject matter
  • The reading environment (Is it noisy? Dirty? Poorly lighted?)
  • The reader’s health, stress, fatigue level, and general state of mind

As a writer, you have no control over these factors—although if you know in advance what these factors are, you can tailor your writing accordingly. For example, job instructions for a piece of factory equipment will probably be written differently from those for a piece of office software.

Factors You Can Control

All external factors being equal, what makes one piece of writing more readable than another? That’s where the internal factors come in. They include, but are not limited to:

  • The writer’s mastery of the language: This is probably the most important factor. Do you know the commonly accepted rules of English grammar? Do you know when it’s acceptable to break them? Do you know the rhetorical tricks that make writing more memorable, pleasurable, or understandable? Do you understand how to use punctuation to improve comprehension? All of these, and dozens more (many of which are discussed in this blog), can make the difference between crystal-clear prose and a garbled mess.
  • The choice of vocabulary: Are you choosing an obscure word because you like it, because you like to show off, or because it is exactly the right word and no other will do?
  • The organization of the passage: Arranging your sentences and paragraphs in a logical fashion, so that there is an orderly flow to your prose, goes a long way towards readability.
  • The writer’s understanding of the intended audience: Frequent readers of this blog have seen this many times: Know your audience! Understand their needs, their motivations, and their limitations.
  • The writer’s understanding of the purpose and subject matter: Do you really know what you’re trying to convey?

Why Not Sentence Length?

The main problem with sentence length as a measure of readability is that it’s so easy to find contradictory examples. A short sentence is may not be more readable than a long one; there are too many other factors at work. A well-written long sentence will be significantly more understandable than a poorly-written short one.   Another issue is that there is no good way to define the threshold between “short” and “long.” There is no “magic number” beyond which readability declines for all readers. Finally, it’s not even that easy to measure sentence length, especially when considering multiple-sentence passages. A paragraph with one 60-word sentence and nine eight-word sentences has the same average sentence length as a paragraph with 10 13-word sentences. Is the former more readable because it has shorter sentences? Or is the latter more readable because it has no long sentences?

Really, people, it’s neither. It’s all in how well those sentences are written. If you want to increase readability, ditch the sentence-length metrics, learn your craft, and understand your audience.

Morris Vaughan is a technical writing consultant in Los Angeles, California.