The only way to know the true effectiveness of a self-defense knife...
We've all seen knife fights in movies and TV shows. The dastardly villain pulls a glistening stiletto, makes some quip about how it has been knife to know our hero, then makes an overhand stabbing motion you'd see coming a mile away. The hero expertly deflects the stab with his forearm, roundhouse kicks the villain into a nearby fruit stand, and rides off into the sunset. Roll credits.
Unsurprisingly, the reality of a knife fight is nothing like this—think less like “The Mask of Zorro” and more like a prison shanking. It's fast, vicious, and often the victim doesn't even realize they've been stabbed until they notice the blood. Knife attacks are so sudden and unpredictable, you probably won't be able to block them effectively, even with considerable training.
Don't just take our word for it, watch this video for a good explanation from martial arts instructor Richard Dmitri (NSFW language):
As you can see, you do not want to go up against an attacker with a knife, especially in close quarters. If your opponent pulls a knife and you're unarmed, your first option should always be to escape. That may sound cowardly to some, but the odds would be stacked against your survival, and running may be the only way to remain unharmed.
If you're sure there's no escape, use kicks or improvised weapons from your surroundings to keep your distance. Attempt to distract or disorient your attacker, then make a run for it when you can. Here's another video that sums up the options effectively:
Despite the myth that most people can effectively block a knife attack with limited training, it's not as easy as some martial arts trainers make it seem. Do yourself a favor: run and live to fight another day.