My knowledge of transistors is pretty limited[0]. Does the yield percentage refer to number of successful chips on a substrate or look more at the total number of successful transistors? (Or confusing hybrid-term like rain forecasts) I believe your comment implies the latter? So the number of successful processors is quite low? How many failed transistors can you have in a working microprocessor? (Probably not an easy to answer question?)

[0] Am I remembering correctly that this is your area?