Embrace the flaws. Embrace them, I say!
Flaw questions on the LSAT are many things, but unpredictable they ain’t – LSAC loves to use variations on the same flaw, over and over. (That’s one of the reasons why the LSAT is such a learnable test.) Obviously, in order to effectively tackle Flaw questions on the LSAT, you should have a good understanding of the flaws themselves. However, it’s also very helpful to know things that are very rarely flaws – things that show up frequently as an answer choice, but are almost never the correct answer.
“The argument presupposes the conclusion it is trying to prove.”
This answer choice describes a circular argument, in which one of the premises is exactly the same as the conclusion. (Note that this is different from premises and conclusions that simply sound similar – students occasionally mistake causal conclusions as being circular, for instance, because they often seem superficially similar to the premises, but causal conclusions have the key difference of introducing causal language.) It frequently shows up on the LSAT, but is rarely the correct answer – in order for something to truly be causal, you should be able to point to a premise that is exactly the same as the conclusion.
“The argument relies on an inappropriately vague usage of [word]” or, relatedly, “the argument relies on two different senses of [word]”.
Arguments that use the same word in two different ways on the LSAT do exist, and those arguments are usually flawed – the classic example is a question in which one person is discussing whether the town can ‘afford’ something (as in, whether they literally have money in the budget), and the other is asking whether the town can ‘afford’ not to do something (as in, the figurative cost of this is too great!).
In order for this to be the correct answer choice, though, an argument literally needs to use the same word in two different ways. Most of the time when you see an answer choice that calls out an argument for using the same word in two senses, that’s not actually what’s happening (or, even more egregiously, the word is only used once in the argument).
“The argument fails to specify [reason for one of the premises].”
When taking the LSAT, you must always assume that the premises are true. The premise of an argument could say something obviously false – like that the sky is green, or that Yankees fans are likeable – and we’d still have to accept it for the purposes of the question. (Flaws on the LSAT occur when the premises fail to properly establish the conclusion, but each individual premise is still considered true!) So answer choices that call out one of the premises for being insufficiently established will never be correct. It is possible that, say, a study does not sufficiently prove what the argument says it proves – but it’s not possible that the results of the experiment as stated in the question are flawed or incorrect.
I’m not trying to say that the above answer choices will never, ever be the right answer. That said, they frequently appear as incorrect answer choices because someone who doesn’t really know what they’re doing will read the answer choice and think, “Yeah, that could be what’s happening here!”
You are not that person. You are an LSAT extraordinaire. So don’t discard the above answer choices out of hand, but do approach with caution if you’re tempted to choose one, and make sure that the situation in the argument matches the answer choice exactly.