Some files are only posted on my web site at specific times when they are needed. Most notably, review sheets for exams in my courses are only posted a week before the exam is to be held, and are removed soon after the exam. This is important because the review sheet must be based on material that was covered in the relevant portions of that course, which can change from one semester to the next as new material is added to the course or outdated material is removed -- a review sheet from a year or two ago may not reflect the current course very well, and may lead you astray in studying for this semester's exam. If you are looking for the review sheet for an exam that is coming up soon (within a week) and it is not here, please contact me -- otherwise, please wait until the week before the exam, so an updated review sheet can be posted that accurately reflects the material that must be reviewed.
I may have moved the requested file to a different directory on the server, renamed the file, or deleted it. I try to keep this from happening very often, and when it does, I try to leave placeholder pages noting the change and directing users to the new file location. It's always possible that I forgot to do this, though, and I can't leave placeholders for non-HTML files (e.g. for copies of old papers or data sets).
It's also possible that the address you tried to access was spelled incorrectly. You may have typed it incorrectly, or you may have followed a link that somebody else typed incorrectly.
You can use the links on the left side of this page to browse through my site to find the page you were looking for. For web pages, the most useful place to look is the Index page, which should list every web page on the site. For data sets or copies of my research papers, your best bets are my Curriculum Vitae, my Data Archive, or my Research page.
You can also try retyping the address, in case there was a typo the last time. Be very careful with both spelling and capitalization -- index.html, index.HTML, Index.html, and INDEX.html are all treated differently by the server (most of my file names are in lowercase), and so are files ending with .htm, .html, and .HTML files (all of mine end with .html).
If all else fails, please email me. As the webmaster for this site, I am the only one who can fix that kind of problem (but probably I won't find out about it for a while unless somebody like you tells me).