The Usual Suspects: Common Logical Fallacies
(Much of this content is identical to the Critical Reasoning section).
There are seven logical errors that appear commonly in the essay questions. When writing your essay argument you should explicitly identify the logical flaw. These flaws also tend to occur in the critical reasoning section of the Verbal GMAT, so your preparation here will benefit you when taking the Verbal section.
1. Circular Reasoning
Here, an unsubstantiated assertion is used to justify another unsubstantiated assertion, which is used to justify the first statement. For instance, Joe and Fred show up at an exclusive club. When asked if they are members, Joe says “I’ll vouch for Fred.” When Joe is asked for evidence that he’s a member, Fred says, “I’ll vouch for him.”
2. The Biased-Sample Fallacy
The Fallacy of the Biased Sample is committed whenever the data for a statistical inference is drawn from a sample that is not representative of the population under consideration. The data drawn and used to make a generalization is drawn from a group that does not represent the whole.
Here is an argument that commits the fallacy of the biased sample:
ln a recent survey conducted by Wall Street Weekly, 80% of the respondents indicated their strong disapproval of increased capital gains taxes. This survey clearly shows that increased capital gains taxes will meet with strong opposition from the electorate.
The data for the inference in this argument is drawn from a sample that is not representative of the entire electorate.