How many words do you know?

http://thelinguist.blogs.com/how_to_learn_english_and/2006/03/how_many_words_.html
Is there one measure of a person's language skill? Are tests like TOEFL TOEIC or IELTS reliable indicators?
In a way the simplest way to find out how well a person uses a language is to ask that person some questions that require lengthy answers, orally or in writing. On oral expose or a written essay will quickly show how the person uses the language. But all of that takes time and the evaluation is necessarily subjective.
So at The Linguist we have a simple scale, one which is easy to measure. It is the number of words known. This can be measured in a cloze test, a test where certain words are blanked out and the learner has to choose from a drop list.
I believe that the number of words known has the advantage of being extremely simple and yet reliable. Most of the learner's skills will correlate to this one index.
We have six levels for our learners
Beginner a) 2,000 b) 3,500 Intermediate a) 5,000 b) 7,5000 Advanced a) 10,000 b) 12,500
Some people may speak better than their known word total, at least in casual social conversation. This means they should read and listen more, work on vocabulary and then write more to increase their vocabulary.
Some people may not speak as well as their word total suggests. They should listen more to content they already understand well. They should focus more on learning phrases. They should work harder on pronunciation. They may also have to write more and then practice reading their corrected writing out loud. If they have the opportunity they should speak more.
Of course each person's case is individual. But the number of words known is still a reliable and yet simple indicator of a person's level. In The Linguist it is an easy number to follow, because we measure vocabulary growth. If a student has 3,000 words then he or she knows what the goal should be. He or she can measure progress towards this goal on a monthly, weekly or even daily basis.
The high frequency words are easier to learn because they are encountered more frequently. On the other hand they are difficult to learn because the learner is not familiar with the language and every words seems strange. Later on the words are encountered less often, but on the other hand the learner is now familiar with relationships between words. Prefixes, suffixes, root words and other similarities between words are easily recognized. Also the learner is better and better able to guess at meaning as the number of unknown words in any text is smaller. So it all balances out.
We count each form of a words as a different word. On this basis I believe a person can learn 25 or more words a day using our system. Most people will learn half that number. This does require, however, a lot of listening to our content on their MP3 players and reading the texts and reviewing words and phrases.