According to Jacob Nielsen, in a post of nearly 500 words, such as this one, readers can be expected to spend an average of about 45 seconds on the page, an amount of time in which they might read some 187 words, or less than three-eighths of the content.
In a study called “Not Quite the Average: An Empirical Study of Web Use,” researchers at the University of Hamburg tracked twenty-five web users’ behavior as they surfed the web as normal. From this data Jacob Nielsen analyzed 45,237 page views of pages with more than 20 words in which the visits lasted longer than 4 seconds and less than 10 minutes. Since users average an additional 4.4 seconds for each 100 words of copy on a page after the first hundred words — an amount of time in which they could be expected to read about 18 words on average — the results suggest that 18 percent of the copy subsequent to the first hundred words is being read.
Nielsen references a scatter chart in making his case:
The following chart shows the maximum amount of text users could read during an average visit to pages with different word counts:
This is a very rapidly declining curve. On an average visit, users read half the information only on those pages with 111 words or less.
In the full dataset, the average page view contained 593 words. So, on average, users will have time to read 28% of the words if they devote all of their time to reading. More realistically, users will read about 20% of the text on the average page.
I think this analysis is flawed, because it aggregates different types of page visits. Some readers are looking for just one piece of information, for example, while others want to follow the full argument of the text. In other words, one interpretation would be that regardless of the length of the copy, a lot of people are only looking for some particular thing. It just takes them slightly longer to find it as it gets surrounded by more verbiage.
I think the lesson to draw here is not that all your web writing should be 100 words or fewer. Rather, it is that if you have items on your website intended for a broad audience — a list of blog policies, for example, or a contact page — you probably want to keep them brief to maximize the amount that will be read by the largest number of visitors. But longer articles could well be the most effective in some ways. With these you are reaching a small group of more dedicated readers.
Short page: many casual readers; long page: few, but dedicated, readers. Some go for brevity, some go for length. For myself, I like a mix of different kinds of content.