<p>You know 18 years in advance when the largest group of 18-year olds will be graduating from HS. When there are lots, it gets more competitive. When the pool is smaller, it gets less. Here are the charts on the trends:
<a href="http://www.collegeboard.com/highered/res/hel/hel.html#grads%5B/url%5D">http://www.collegeboard.com/highered/res/hel/hel.html#grads</a></p>