To this we need the statistics, the studies with large samples (ie possibility of observing the phenomenon) and the multicentric ones (which take place in different places). This is because my donkey could fly because he suffers from a malformation that gave him this possibility: my observation therefore would not be equivalent to what the donkeys fly but that only one of them fly.
But there is a loophole to these lengthy procedures. The evidence.
If I can not show everyone my donkey that flies because he is shy and sometimes he refuses, if he does not repeat his flights because on cloudy days he is sad and refuses to jump, if he does so in a very confused way It is awkward that a measurement is impossible, but it flies, and I think this is certain because I find it one day on one tree, the other on the third floor of a building and the next day on an inaccessible tower, there is evidence. However, the evidence must be checked: does someone carry that donkey above the tree or the tower? Is it me that with a powerful jet of air or with ropes lift the donkey up to make it rise up? In short, is the evidence a real fact or is it just a trick or imagination?
If we exclude fraud and conditioning (it could be all a dream), the evidence is certain: that fact takes place.
We do not know how and why, nobody can prove it scientifically but that donkey flies, there is little to do. Until proven otherwise.