I couldn’t lecture half as well in a studio as I do live–I need the audience to get in the flow, the bigger the better–so I don’t foresee ever doing 10-hour videos.

@doug and jonathan: in my experience it takes 5-10 hours to write down lecture notes for a 90-minute class, but it takes only 1-2 hours if I don’t write notes, so it’s in the ballpark of the factor of 4 of Jonathan’s. I have been told that it takes about 10 hours to produce videos with the content of a 90-minute class (not counting editing, uploading etc which is done by a ta) so there is no advantage there.

I think that it is easier to learn advanced material by taking a class than by reading a book: part of it is that a proof presented verbally has a different “flow” from the way it is usually written down (although I make an effort, in papers and notes, to write proofs as I would describe them verbally); part of it is the schedule of two classes a week, that keeps you moving forward, while on your own it’s easier to get sidetracked. The online class format would recreate those advantages for the non-Stanford students taking the class. For the Stanford students, there is the “flipped classroom” model, of which I am skeptical, but people that know about teaching more than I do swear by its effectiveness.

Doug: My personal experience is that it does indeed take less time to prepare a lecture than written material; for me, by about a factor of four. I suppose much of this gap is because of higher standards for the written material; e.g. machine-drawn figures instead of hand-drawn, formal prose instead of conversational. If I had to write out all my graduate lectures so they could stand alone, I’d never get them done on time. Though I agree with you that it would be better if I could.