Document analysis is a somewhat different concept than user engagement, but it can be used to predict how users will perceive the quality of content on a site. Strong signals indicating a likelihood of poor-quality content could potentially be used as a ranking signal by the search engines.
Poor spelling and bad grammar can indicate a lack of attention and care about the content. Care must be exercised here, as user-generated content will not necessarily be checked for spelling or grammar. Publisher-authored content that is not subject to quality editing, however, may be an indication of poor quality.
The concept of reading level refers to the level of the sophistication of the writing—that is, what grade level a reader would have to have attained in order to understand the content. One formula for this is known as the Flesch-Kincaid Reading Test. It measures the average number of words per sentence and the average number of syllables per word; it is actually not a measurement of the education level required to read the content, and might be better thought of as a measurement of the mental effort required to read a sentence.
If you want to check the reading level of a document, you can do that using the spelling and grammar checker in Microsoft Word (http://office.microsoft.com/en-us/word-help/test-your-document-s-readability-HP010148506.aspx). You can also configure the Advanced Search settings (http://www.google.com/advanced_search ...