Researcher: Each subject in this experiment owns one car, and was asked to estimate what proportion of all automobiles registered in the nation are the same make as the subject's car.
Conclusion
Certain makes of car must be more common in some areas than others.
Evidence
If that were so, then many people would overestimate how common their type of car is. + In a study, many people overestimated how common their type of car is.
Evaluation
This is committing a Necessary vs. Sufficient flaw by doing backwards logic on a conditional. If my hypothesis were true, we would see X. We see X, thus my hypothesis is true.
Here's a parallel argument: I hypothesize that people think their own lives are typical of other people's lives. If this were so, then many people would overestimate how typical their car is. Since people in this study overestimated how typical their car is, it must be that people think there own lives are typical of others'.
Goal
Look for an answer pointing out that there are other possible ways to interpret the fact that people overestimate how common their car is, or an answer calling out the backwards logic (Nec vs. Suff) move the author made.