I am supposing that "real knife attack" means "attack with a knife with intent to harm" as opposed to "attack with a knife by a knife expert".
Tanto-dori techniques in aikido are being criticized as being of dubious effectivity against an attacker trained in a knife fighting system (eg., some style of Filipino Martial Art or SIlat, or the Fairbairn knife fighting method, etc). However, what is the reality? What are the odds that when we do meet a knife wielding attacker, said attacker will be someone trained in the aformentioned arts?
At least if we have some training in the concepts of Ma-ai, irimi, aiki and atemi, we will have a slightly better chance than if we had no training at all.
I would really like to see a retrospective study of knife attacks to find out the training history (if any) of the attackers, as well as how the vicims defended themselves and the results..
Sounds like a nice search on the Pubmed database