We follow the same principle, but using only a small sample of words (around 120) to achieve the same result. Among all the words you check in the second step, we find which word (say, #55) has the same number of blank checkboxes before it (say, 18), as it has checked boxes after it (again, 18). We then look up the frequency rank of this "midpoint" word #55, which turns out to be #15,000, which means that you know 15,000 words.
Here is my question:What I can't understand is " we find which word (say, #55) has the same number of blank checkboxes before it (say, 18), as it has checked boxes after it (again, 18)."..Mainly I don't understand "which" and "it" mean what.
And the previous paragraph is ------To understand how we come up with the exact number at the end, let's start with an analogy. Imagine you have the whole dictionary of 45,000+ words, with words arranged in order from most-common to least-common, and you mark all the words you know. At the end, you go back, and discover that at exactly word #15,000, there are 2,000 words that came earlier (more common words) which you didn't know. And at word #15,000, there are 2,000 words which come afterwards (less common words) which you do know. The 2,000 after which you do know cancel out the 2,000 before you don't, and in the end it means you know 15,000 words.