We've all heard this ever. Some may even have used this argument: "it works for me." It seems that it works as a defense of the effectiveness of everything. From homeopathy to ouija including NLP or MMS. If you've tried it and you've seen that it works and maybe you even know more people that say that it works, we could say that its effectiveness is proved, right? Let's see why this argument could be wrong.
Our perception of efficacy is affected by certain psychological and sociological biases that prevent us from correctly assessing the results and even alter them unconsciously. This is a well-known phenomenon and this is why personal experience is a questionable way of generating knowledge. Instead, we use studies carried out using the scientific method that has been designed to attempt to correct and neutralize these biases in order to try to get results as objective and solid as possible. Also, if the results resist falsification tests and replication by different teams, we ensure a more consistent result.
For greater clarity when explaining why personal experience is not a valid proof, we are going to review with a little more detail some of these biases with examples of how they affect the assessment of an outcome. It should be clear that all we apply these biases, mostly unconsciously. So we can't assume bad faith on those who fall into these biases and can't either consider ourselves free of them.
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Source: Amazon Confirmation bias. Probably one of the best known and studied. It may be referred to by many other names or subtypes, some of which are also explained below, like recall bias, cherry-picking, biased interpretation, etc. It is the tendency we have to give more value to cases that confirm our point of view. This applies, for example, if we take a homeopathic preparation every time we have a cold and the third part of the times we do it, we are healed. If we tend to believe that homeopathy works, we will give more validity and representativeness to that cases where we've cured even though it is less the times —and however, it may not be representative even if they were most of the cases, as we will see below. It is the unconscious tendency we have to give more relevance to cases that favor our point of view and disregard those who contradict it.