Once you have mastered the art of finding the central assumption of a critical reasoning problem, the next skill to focus on is identifying ways in which to strengthen or weaken an argument. As strengthen and weaken questions are even more prevalent on the GMAT than assumption questions, the ability to identify ways to strengthen and weaken the argument is critical to answering a majority of critical reasoning questions correctly.
The key to strengthening an argument is finding the answer choice that confirms that the central assumption is correct. This is generally done in one of two ways. First, the correct answer can simply be a rewording of the assumption itself. If you encounter an answer choice that would be correct on an assumption question, it would also be correct on a strengthen question. Second, the correct answer can confirm the assumption by citing a study, survey or any other relevant piece of additional evidence that makes the assumption more likely to be true. Remember, you only need to make the argument more likely to be correct; you do not need to make the argument certain to be correct.
Finding a statement that weakens an argument follows the same process, only backwards. Again, two possibilities exist. First, an answer that weakens the argument can directly refute the assumption. Any choice that states that the assumption is wrong will weaken the argument. Second, just as new pieces of evidence can strengthen the argument, new pieces of evidence can weaken the argument. Any new information presented in an answer choice that makes the assumption less likely to be correct will act to weaken the argument as a whole.
By knowing the two common types of strengtheners and weakeners, you will be able to quickly identify correct answers in strengthen/weaken questions and be on your way to an even higher GMAT score.