Many, if not most, of these classes (note I said class and NOT course), are excerpts from a full course. DO NOT get hooked into buying copies of these that you already may have from the full (and sometimes the full ones are actually missing pieces) course. Note that CL does not state that these are excerpts from other full courses. I bet, too, that some of them may actually not be in the course content of a full course. I have bought some full ones where the instructor says that a topic will be covered the next day or so but is never mentioned. Caveat Emptor, viewers.