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50% is a debating rule which states that in any conversation about a topic with differing opinions, disregarding expertise, knowledge and experience, a person is generally 50% likely to be correct1). It's just statistics. Keep in mind that, in the real world, these odds are affected by expertise and many other factors, such as the generality of your claim (the more general the claim, the less likely you are to be correct, except in some exceptions), the temperament of an argument (the more emotional an argument, the less likely anyone is to be correct), and more.
If a person never admits to being wrong, never backpedals on one of their claims, aggressively defends each and everything they say, even the most fringe and extreme things, there are some great odds that you are talking to someone who just doesn't admit to being wrong even when they are. It indicates that your argumentation isn't the fault (unless of course they do backpedal on their claims when talking to someone else, in which case they may be biased against you as a person, biased in favor of that other person, or your arguments really do just suck). Of course, this doesn't mean that you are more correct than them, it definitely cannot be used predictively to tell you exactly when you are wrong and when you are right, but it can be used as a general guideline to help understand whose criticisms to take serious, and how much so, even without having a single bit of expertise in any field of study.
On the other end, it helps you guide your understanding of yourself, recognize where, when and why you may be more wrong about topics than you expect, or primarily realize that you probably are wrong in the first place.