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…