Research methodologies cannot be the same for all types of phenomena. Everything is not physics or chemistry or biology. Professionals in certain areas know what is a significant piece of evidence in that area.
Indeed it is - and this kind of research uses the tools and techniques of psychology research. Now I'm not a psychologist, but I do understand the methods pretty well as I used to be in overall change of research for a faculty that included our psychology department. In addition I've been on research ethics assessment panels for years and seen countless psychology research proposals. What I can say is that the research here is very poor methodologically - and indeed Tucker even admits this in his book on the topic - for example the approach to showing pictures to the child. You just don't do it the way he did. You need someone completely independent and 'blinded' (i.e. has no knowledge themselves of which person, or any people are relevant). If you do it as he did - researcher who knows who the key person is showing sets of four pictures each of which include a key person, you will get subliminal cues that are picked up by the research subject which biases the outcome. Also you will always have a one in four hit. You also need some 'control' groups of photos, in which there is no relevant person - this unpicks whether the child (in this case) is simply always picking a person as they think this is the 'game' - if they continually say they know a person from a group of four completely random people it shows that they aren't actually basing their choices on real knowledge.
And there are all sorts of elements of this kind.
Now if you read the book one thing is striking - virtually all the questions are answered by the mother, not the child - the information is being filtered through the prism of the mother. There is also a Netflix piece on this case in which Ryan is deeply embarrassed about the whole thing and all the information comes from the mother. I'd defy anyone to watch this and conclude anything other than the mother has a huge amount of involvement in the claimed reincarnation.
There is an excoriating bit in the Neflix piece where Martyn's neice (pretty well the only person to have known Martyn as an adult as he died when his daughter was just eight). She asks him a couple of quite specific questions which clearly relates to important things in Martyn's life - e.g. Lindburgh landing in Paris when he was there. Ryan is completely flummoxed - he simply has nothing to say.
Finally it is claimed that of 200 statements Ryan gets about 50 correct on Martyn's life. Well firstly this isn't particularly impressive. But also many of those claims he is right about were available as public knowledge at the time (e.g. he was a dancer, he lived in a house with Rox in the name, his children etc, etc), but many of the others are so anodyne to almost certainly be the case, for example:
He was very rich - no shit Sherlock - it was common knowledge that his wife was the daughter of a movie company executive.
He smoked - no shit Sherlock - pretty well everyone smoked in those days
He wore a hat - no shit Sherlock - pretty well everyone did in those days
His house was big and had a pool - well a quick internet search brings up the house (common knowledge) and its details
His house had a brick wall - really, I mean really!!
He drove a green car - well if you asked me (or probably you) whether you drove a green car the answer is yes, but then it is also yes if you asked about a blue, red, silver etc car. And of course in those days the colours of cars were more limited so it is hardly earth shattering that between him and his wife they drove green and black cars (the most common colours).