According to this link, a study proved that "90 percent of a person's traits can be judged with their shoes." Without needing to look at the study, a reader can reason that this claim makes no sense.
What does it mean by 90 percent of a person's traits? How many traits are there in a person? If the researcher defines 1000 traits, then the shoe predictor will need to predict 900 traits accurately. If the researcher defines 10 traits, then the shoe predictor need only predict 9. Needless to say, the demands on the predictor are vastly different in those two cases.
What is a trait? According to the article, examples are "age, gender, income, political affiliation, emotional and other important characters". Further, the article lists a number of "general observation results", including such gems as "Shoes that were not new but appeared to be spotless belonged to conscientious types". So, conscientiousness is a trait that can be predicted by observing our shoes.
But what does it mean by conscientiousness? It appears that in this study, any trait is a binary variable. A person is either conscientious or not - there is no shade of gray.
In fact, the accuracy of the prediction is also black and white - there is no shade of gray. Each trait is either judged or not judged with shoes. That's how you get to 90 percent. But it is simply impossible that a shoe predictor will correctly identify every single conscientious person. Even if we allow that any person can be described as 100% conscientious.
What if conscientiousness were treated as a rating (say, 1 to 10) instead of a binary variable? Then, we'd like to know what it means by accurate prediction. Is it that the shoe predictor must get the right level out of 10 or is it getting "close enough"? What is "close enough"?
How was the research conducted? Volunteers have their shoes photographed, and they fill out a questionnaire about their traits (hmm, maybe something like "I'm a dishonest human being"?). Study subjects (students, as is often the case in these studies) are asked to review pictures of shoes and to predict the traits of their owners.
So, in fact, the students did not predict traits, they predicted the self-evaluation of volunteers.
Look at the study conclusion again: "Shoes that were not new but appeared to be spotless belonged to conscientious types." The students have to look at the shoes and then guess that the owner is conscientious. The statement is written as if it is a law of nature. Surely, not everyone who wears spotless, old shoes are conscientious. What proportion of these owners are conscientious? If this percentage is say 60%, then what does it mean to say students predicted conscientiousness correctly?