How many words do you need to know ?

Research indicates that if the students know about 98% of the words on a page, then they can read it quickly and with high levels of comprehension. Below 90% (one unknown word in 10) the reading becomes frustrating and slow requiring a lot of dictionary use and comprehension suffers badly.
The Extensive Reading Foundation’s Guide to Extensive Reading

I first came across this research while teaching English 109, and it sure corresponds to my experience. 

If my students knew just 90% of the words in a text, they couldn’t read it at all, even with the definitions of new vocabulary words typed in the margins. 

I’m sure the problem is working memory.

Every time you look up a new word you have to remember a) what you were just reading, b) the ‘name’ of the new word, and c) the new word’s definition.

Plus you have to moosh all these things together into a meaningful whole, an operation that is also performed by working memory.

And since you know only 90% of the words, you have to do this over and over again, which erodes your memory of the other words you just looked up, not to mention your memory (and, consequently, your understanding) of the essay as a whole. 

It’s like trying to multiply 79 by 6 inside your head. You have to remember the 79, you have to remember the 6, you have to remember the subproducts (is that the right word ? I don’t remember !), and you have to perform the calculation. 

It’s too much.

This is why schools should teach vocabulary.