Which logical fallacy is used in the text?
Our study looked at people who were successful, as defined by their level of income. Our findings show that successful people consistently make more money than unsuccessful people.

 - false causation: the assumption that because two things happened together, one caused the other
 - circular reasoning: an argument that supports a claim with the claim itself
 - ad hominem: an attack against the person making the argument, rather than the argument itself
The text argues that successful people make more money than unsuccessful people. However, the study defines success by income level. So, the "evidence" is just a restatement of the claim itself. This illustrates a type of logical fallacy known as circular reasoning.