Most subdirectories of this directory are for courses I've given or
am giving, e.g. 443 is for 18.443. If it's just a 3-digit number,
it's a course I'm currently giving or the site from the last
time I gave the course and is openly accessible.

If instead there's an indication of a past date, such as 465S05,
it's materials for a past time I gave the course, not the latest and
often not up to date. Some of these
were incorporated into OpenCourseWare sites, which exist for 18.465 
(nonparametrics and robustness, Spring 2005), and 18.466 Mathematical
Statistics. These "old" subdirectories are not accessible except to
me.