The problem ISN'T that students don't have the strategies and skills to understand a text. The problem ISN'T that they don't know how to skim and scan or make predictions about a text. The problem is that they understand the text only at the word level, but not what the whole thing means or how the ideas fit together. All of their brain power, their working memory, is devoted to understanding the words and there's no spare capacity for general understanding of what it means.

Higher levels of understanding only arise when lower levels have been firmly understood

Higher levels of understanding only arise when lower levels have been firmly understood and are in long term memory. For this reason, you can't do higher level math if you can't do basic arithmetic almost instantly.

In the same way, if you don't know the meaning of several key words in a sentence, this slows down and limits comprehension. Working memory is limited to about 3-5 items. With a challenging reading passage, working memory is maxed out on the concepts and the vocabulary; every new idea that the young reader struggles to understand PUSHES OUT another idea about the text that was in working memory. 

Working memory is a juggler with a fixed number of arms

Think of working memory as a juggler with a fixed number of arms. The items being juggled are all the ideas that your brain can handle at once. Any more, and you feel overwhelmed; you can't keep all the ideas in mind at once and you start dropping items you're juggling. Since working memory is limited, how do young readers overcome this issue? Trying to solve this issue through reading comprehension skill building is looking at the problem through the wrong end of the telescope. The real cause of the problem is lack of background knowledge and vocabulary. According to professor Michael Swan, "When decoding gets more automatic, [students will] be able to access higher level comprehension skills." Swan believes stand-alone reading comprehension activities are a waste of time. 

Reading or listening comprehension = comprehension

Reading or listening comprehension is not a skill. Reading or listening comprehension = comprehension. It's not that you don't understand a text because you lack a skill; you don't understand a text because the text is composed of too many smaller things you don't understand well enough.

A clever experiment was performed to test whether reading comprehension depends more on reading ability or on background knowledge. A model baseball game was set up with figurines to represent the players. The reading comprehension test takers had to read the text and then move the figurine players around to show what happened in the text. Those with the high report card English scores but little knowledge of baseball scored worse than those with low English scores but a lot of knowledge about baseball.

Understanding non-fiction texts is therefore harder than understanding stories. You don't need much background information to understand a story, but non-fiction texts usually require some specialized knowledge, such as from history, geography, or science. Content knowledge will help you understand such texts, not reading comprehension skills.

Complex sentences cause confusion

Lack of understanding of complex sentence structures is another cause of misunderstanding. Long sentences with embedded clauses cause confusion for inexperienced readers because it can be hard to figure out which part of the sentence goes together with which other part of the sentence. As a result, working memory is again maxxed out when it's focused on solving the sentence structure puzzle instead of on meaning. Swan gives an example of a reduced relative clause:

• The children asked for comments said they disagreed.

"Asked for comments" is a reduced form of "who were asked for comments." To an inexperienced reader (or listener) who isn't familiar with such relative clause reductions, it seems that the children were the ones asking for comments, which, of course, doesn't make sense.

Without knowledge of such rather sophisticated grammar constructions, and there are many others, inexperienced readers will always struggle to understand complex texts.

For these reasons, working on reading comprehension skills on their own is largely a waste of time.